Optical system, finder optical system, relay type finder optical system, eyepiece optical system, and single-lens reflex camera

ABSTRACT

The invention concerns an optical system adapted to split an optical path from a taking optical system into a finder optical system, which has a camera-shake correction function best suited for the finder optical system, and so on. This optical system comprises a taking optical system and a finder optical system. The taking optical system comprises an optical path splitter means operable to split an optical path into the finder optical system, and the finder optical system includes a relay optical system RLS for re-forming a subject image once. Anti-shake is implemented by shifting an optical subsystem RL that forms a part of the finder optical system in a plane vertical to an optical axis.

This application is a continuation of prior U.S. patent application Ser.No. 11/322,641, filed Jan. 3, 2006, and also claims benefits of JapaneseApplication No. 2005-3395 filed in Japan on Jan. 11, 2005 and JapaneseApplication Nos. 2005-79097, 2005-79098, 2005-79099 and 2005-79100 filedin Japan on Mar. 18, 2005, the contents of each of which areincorporated by this reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to an optical system, a finderoptical system, a relay type finder optical system, an eyepiece opticalsystem and a single-lens reflex camera, and more specifically to anoptical system having a camera-shake correction function capable ofpreventing image-shakes by camera-shake (e.g., upon hand-holdphotographing), a relay optical system designed especially for takingsystems such as digital cameras, film cameras and video cameras, aneyepiece optical system having a wide field of view, a long eye reliefand a compact total length, a compact relay type finder optical systemusing the same, and a single-lens reflex camera using the same.

Prior art finder optical systems having a camera-shake correctionfunction include the following.

The finder optical system set forth in patent publication 1 isconfigured as shown in FIG. 45. In this optical system, the camera-shakecorrection lens L is located in a non-TTL finder optical system whereinthe taking optical system is separate from the finder optical system.Then, the camera-shake correction lens L is decentered parallel with theoptical axis of the finder optical system by means of a driving systemA, thereby implementing correction of shakes of light incident from asubject on the optical system.

The optical system set forth in patent publication 2 is configured asdepicted in FIGS. 46, 47 and 48. In FIGS. 46 and 47, the lens L thatforms a part of the objective lens is moved vertically to the opticalaxis, and in FIG. 48, the eyepiece lens E is moved vertically to theoptical axis. In this way, correction of camera-shake is implemented.

With the above prior arts, a lens group having a camera-shake preventivefunction, i.e., an anti-shake group is used. As this anti-shake group isdecentered, it produces aberrations that lead to image-formationcapability deteriorations. It also produces decentration distortion thatmay otherwise cause an image shape to turn asymmetric with respect tothe optical axis. There is thus the need of preventing suchimage-formation capability deteriorations and asymmetric image shape dueto anti-shake

Further, as the anti-shake group is heavy, it gives an increasing loadto an actuator for the movement of the anti-shake group; that is, it isnecessary to lighten the anti-shake group.

Furthermore, a low sensitivity to anti-shake causes an increase in theamount of movement of the anti-shake group. A high sensitivity toanti-shake, in contrast, renders control of movement of the anti-shakegroup difficult. To eliminate this problem, the sensitivity toanti-shake must be set at a proper value.

With anti-shake optical systems, much care must thus be taken ofanti-shake functions.

The optical systems disclosed in patent publications 1 and 2 teachnothing about how the problems to be solved with anti-shake opticalsystems are addressed. In other words, these prior arts are still lessthan satisfactory in terms of practically optimum anti-shake methods,and how they are feasible. In particular, some patent publications donot present any optical path diagrams.

The optical systems disclosed in patent publications 1 and 2 are alldirected to a non-TTL finder optical system with a taking optical systemseparate from a finder optical system.

By the way, single-lens reflex cameras comprising an image pickup planesmaller than conventional Leica-format size have been proposed andcommercialized. Especially the market for single-lens reflex camerasusing CCDs, C-MOSs or other electronic image pickup devices are nowexpanding. Decreased image pickup planes need a finder system of highermagnification. Although it is necessary to shorten the focal length ofthe whole finder optical system so as to make finder magnification high,this renders it difficult to set up a finder system in a widelyavailable penta prism mode. Another technique known in the art is to usea relay type finder optical system, as typically taught in patentpublications 3 and 4. Never until now, however, is there proposed anyrelay optical system, and any eyepiece optical system that is of goodperformance, is suitable for use with a small image pickup lane, and canbe compactly laid out.

Further, there is still desired an eyepiece optical system or lens thatensures a relatively wide field of view, a long distance (eye relief)from the eyepiece lens the viewer's pupil (eye point), and a compactwhole length inclusive of the position of an image being viewed as setby an image-formation lens, and a display position. Such eyepieceoptical systems as set forth in patent publications 5 and 6 have so farbeen available. However, although they have a sufficiently wide angle ofview, yet they are of inadequate eye relief.

Still, an eyepiece optical system having a long eye relief is desired inview of camera body construction as well.

Patent Publication 1

JP(A))9-329820

Patent Publication 2

JP(A)2003-91027

Patent Publication 3

JP(A)4-337705

Patent Publication 4

JP(A)1-101530

Patent Publication 5

JP(A)8-43749

Patent Publication 6

JP(A)9-54258

SUMMARY OF THE INVENTION

The prior art situation being like this, one object of the invention isto provide an optical system operable to split an optical path from ataking optical system into a finder optical system, wherein acamera-shake correction function best suited for the finder opticalsystem is provided.

Another object of the invention is to provide a compact relay typefinder optical system that is compatible with a single-lens reflexcamera using a relatively small image pickup device, and a single-lensreflex camera incorporating it.

Yet another object of the invention is to provide an eyepiece opticalsystem that has a relatively wide field of view, a long distance (eyerelief) from an eyepiece lens to a viewer's pupil (eye point) and acompact total length including an image viewing position by animage-formation lens.

A further object of the invention is to provide an eyepiece opticalsystem that is compatible with a single-lens reflex camera using arelatively small image pickup device and suitable for use as a compactrelay type finder optical system, and a finder optical systemincorporating it.

The present invention, with which the above objects are achievable, isgenerally broken down into five aspects, of which the first aspect isdirected to the following optical system.

An optical system, comprising:

a taking optical system, and a finder optical system, wherein:

the taking optical system comprises an optical path splitter meansoperable to split an optical path into the finder optical system,

the finder optical system comprises a relay optical system operable toform a subject image once, and

the finder optical system comprises an optical subsystem that isoperable to shift for anti-shake in a plane vertical to an optical axisof the finder optical system, and forms a part of the finder opticalsystem.

The second aspect of the invention includes the following finder opticalsystems and single-lens reflex cameras.

[1] A finder optical system, comprising:

a relay optical system operable to re-form a subject image at asecondary image-formation position wherein the subject image is aprimary image formed at a primary image-formation position through anobjective optical system, and

an eyepiece optical system operable to view an image re-formed throughthe relay optical system, wherein:

the finder optical system comprises at least four reflecting surfacesincluding an F3 reflecting surface, an F2 reflecting surface, an F1reflecting surface and an R1 reflecting surface located between theprimary image-formation position and the secondary image-formationposition side and in an optical path order from a primaryimage-formation position side to a secondary image-formation positionside, and at least one positive lens located between the F1 reflectingsurface and the R1 reflecting surface,

the finder optical system has an optical axis reflected at each of thereflecting surfaces, and

when a direction of a light ray traveling on an optical axis is taken asan optical axis direction and a component parallel with an optical axisdirection exiting the eyepiece optical system is taken as a transversecomponent,

a transverse component in an optical axis direction exiting from the F2reflecting surface is opposite to an optical axis direction exiting theeyepiece optical system, and

a transverse component in an optical axis direction exiting from the R1reflecting surface is opposite to the optical axis direction exiting theeyepiece optical system.

[2] A finder optical system, comprising:

a relay optical system operable to re-form a subject image at asecondary image-formation position wherein the subject image is aprimary image formed at a primary image-formation position through anobjective optical system, and

an eyepiece optical system operable to view an image re-formed throughthe relay optical system, wherein:

the finder optical system comprises at least four reflecting surfacesincluding an F3 reflecting surface, an F2 reflecting surface, an F1reflecting surface and an R1 reflecting surface located between theprimary image-formation position and the secondary image-formationposition side and in an optical path order from a primaryimage-formation position side to a secondary image-formation positionside, and at least one positive lens located between the F1 reflectingsurface and the R1 reflecting surface,

the finder optical system has an optical axis reflected at each of thereflecting surfaces, and

when a direction of a light ray traveling on an optical axis is taken asan optical axis direction and a component parallel with an optical axisdirection exiting the eyepiece optical system is taken as a transversecomponent,

a transverse component in an optical axis direction incident on the F1reflecting surface is opposite to an optical axis direction exiting theeyepiece optical system, and

a transverse component in an optical axis direction exiting from the R1reflecting surface is opposite to an optical axis direction exiting theeyepiece optical system.

[3] A finder optical system, comprising:

a relay optical system operable to re-form a subject image at asecondary image-formation position wherein the subject image is aprimary image formed at a primary image-formation position through anobjective optical system, and

an eyepiece optical system operable to view an image re-formed throughthe relay optical system, wherein:

the finder optical system comprises at least four reflecting surfacesincluding an F3 reflecting surface, an F2 reflecting surface, an F1reflecting surface and an R1 reflecting surface located between theprimary image-formation position and the secondary image-formationposition side and in an optical path order from a primaryimage-formation position side to a secondary image-formation positionside, and at least one positive lens located between the F1 reflectingsurface and the R1 reflecting surface,

an optical axis is acutely reflected at the F1 reflecting surface andacutely reflected at the R1 reflecting surface, and

an angle that the F1 reflecting surface subtends the R1 reflectingsurface is an acute angle, provided that when at least one of the F1reflecting surface and the R1 reflecting surface is a curved reflectingmirror, the subtending angle is an angle that tangent planes subtendeach other at a position of incidence of an optical axis.

[4] A single-lens reflex camera, comprising:

a light beam splitter means operable to split light beams incidentthrough an objective optical system into a light beam incident on animage pickup device and a light beam incident on a finder optical systemoperable to bend a light beam to view a subject image, and

a finder optical system operable to bend a light beam from the objectiveoptical system to view a subject image, wherein:

the finder optical system comprises a focal plane plate located at aprimary image-formation position set at a surface optically equivalentto the image pickup device,

a relay optical system operable to re-form a subject image at asecondary image-formation position wherein the subject image is aprimary image formed at the primary image-formation position, and

an eyepiece optical system operable to view an image re-formed throughthe relay optical system, and wherein:

the finder optical system further comprises:

an F1 reflecting surface, an R1 reflecting surface and an R2 reflectingsurface located in an optical path order from a primary image-formationposition side to a secondary image-formation position side, and

at least one positive lens located between the F1 reflecting surface andthe R1 reflecting surface, wherein:

an optical axis of the finder optical system is reflected at each of thereflecting surfaces, wherein an optical axis is acutely reflected at thelight beam splitter means, an optical axis is acutely reflected at theF1 reflecting surface, an optical axis is acutely reflected at the R1reflecting surface, an angle that the F1 reflecting surface subtends theR1 reflecting surface is an acute angle, provided that when at east oneof the F1 reflecting surface and the R1 reflecting surface is a curvedreflecting mirror, the angle that the F1 reflecting surface subtends theR1 reflecting surface is given by an angle that tangent planes subtendeach other at a position of incidence of an optical axis, and

an optical axis of the eyepiece optical system is substantially parallelwith an optical axis of the objective optical system.

The third aspect of the invention includes the following finder opticalsystems.

[1] A finder optical system, comprising:

a relay optical system operable to re-form a subject image at asecondary image-formation position wherein the subject image is aprimary image formed at a primary image-formation position through anobjective optical system, and

an eyepiece optical system operable to view an image re-formed throughthe relay optical system, wherein:

the relay optical system comprises an FP lens group of positiverefracting power, an N lens group of negative refracting power and an RPlens group of positive refracting power, wherein:

the FP lens group of positive refracting power consists of a first lenshaving positive refracting power, and a second lens located on a side ofthe first lens facing the eyepiece optical system and having positiverefracting power,

the N lens group of negative refracting power consists of a third lensgroup located on a side of the FP lens group facing the eyepiece opticalsystem and having negative refracting power, and

the RP lens group of positive refracting power consists of a fourth lenslocated on a side of the N lens group facing the eyepiece optical systemand having negative refracting power and a fifth lens located on aneyepiece optical system side with respect to the fourth lens and havingpositive refracting power, and the relay optical system satisfies thefollowing condition with respect to a composite focal length, flr, ofthe EP lens group, the N lens group and the RP lens group, and an axialdistance, dl, between the primary image-formation position and a surfaceof the first lens on a primary image-formation position side:

0.3<dl/flr<3  (3-1)

[2] A finder optical system, comprising:

a relay optical system operable to re-form a subject image at asecondary image-formation position wherein the subject image is aprimary image formed at a primary image-formation position through anobjective optical system, and

an eyepiece optical system operable to view an image re-formed throughthe relay optical system, wherein:

the relay optical system comprises an FP lens group of positiverefracting power, an N lens group located on a side of the FP lens groupfacing the eyepiece optical system and having negative refracting power,and an RP lens group located on a side of the N lens group facing theeyepiece optical system and having positive refracting power, wherein:

the RP lens group consists of, in order from a primary image-formationposition side to a secondary image-formation position side, a negativelens and a positive lens, and

the relay optical system satisfies the following condition (3-2) withrespect to an axial distance, drf, from a surface located in, andnearest to a secondary image-formation position side of, the FP lensgroup to a surface located in the N lens group and on a secondaryimage-formation position side and an axial distance, drr, from a surfacelocated in the N lens group and on a secondary image-formation positionside to a surface located in, and nearest to a primary image-formationposition side of, the RP lens group:

0.2<drf/drr<0.8  (3-2)

[3] A finder optical system, comprising:

a relay optical system operable to re-form a subject image at asecondary image-formation position wherein the subject image is aprimary image formed at a primary image-formation position through anobjective optical system, and

an eyepiece optical system operable to view an image re-formed throughthe relay optical system, wherein:

the relay optical system comprises an FP lens group of positiverefracting power, an N lens group of negative refracting power and an RPlens group of positive refracting power, wherein:

the FP lens group of positive refracting power consists of a first lenshaving positive refracting power, and a second lens located on a side ofthe first lens facing the eyepiece optical system and having positiverefracting power,

the N lens group of negative refracting power consists of a third lensgroup located on an eyepiece optical system side with respect to the FPlens group and having negative refracting power, and

the RP lens group of positive refracting power is located on an eyepieceoptical system side with respect to the N lens group, and

the relay optical system satisfies the following conditions (3-1) and(3-3) with respect to a composite focal length, flr, of the EP lensgroup, the N lens group and the RP lens group, an axial distance, dl,between the primary image-formation position and a surface of the firstlens facing the primary image-formation position, and an axial distance,ds, from a surface located in the RP lens group and on a secondaryimage-formation position side to the secondary image-formation position:

0.3<dl/flr<3  (3-1)

0.5<ds/flr<2  (3-3)

[4] A finder optical system, comprising:

a relay optical system operable to re-form a subject image at asecondary image-formation position wherein the subject image is aprimary image formed at a primary image-formation position through anobjective optical system, and

an eyepiece optical system operable to view an image re-formed throughthe relay optical system, wherein:

the relay optical system comprises an FP lens group of positiverefracting power, an N lens group of negative refracting power and an RPlens group of positive refracting power, wherein:

the FP lens group of positive refracting power consists of a first lenshaving positive refracting power, and a second lens located on a side ofthe first lens facing the eyepiece optical system and having positiverefracting power,

the N lens group of negative refracting power consists of a third lensgroup located on an eyepiece optical system side with respect to the FPlens group and having negative refracting power, and

the RP lens group of positive refracting power consists of a fourth lenslocated on an eyepiece optical system side with respect to the N lensgroup and having negative refracting power, and a fifth lens located ona side of the fourth lens facing the eyepiece optical system havingpositive refracting power, and the relay optical system satisfies thefollowing conditions (3-1), (3-2) and (3-3) with respect to a compositefocal length, flr, of the EP lens group, the N lens group and the RPlens group, an axial distance, dl, between the primary image-formationposition and a surface of the first lens on a primary image-formationposition side, an axial distance, drf, from a surface located in, andnearest to a secondary image-formation position side of, the FP lensgroup to a surface located in the N lens group and on a secondaryimage-formation position side, an axial distance, drr, from a surfacelocated in the N-lens group and on a secondary image-formation positionside to a surface located in, and nearest to a primary image-formationposition side of, the RP lens, and an axial distance, ds, from a surfacelocated in the RP lens group and on a secondary image-formation positionside to the secondary image-formation position:

0.3<dl/flr<3  (3-1)

0.2<drf/drr<0.8  (3-2)

0.5<ds/flr<2  (3-3)

The fourth aspect of the invention includes the following eyepieceoptical systems and relay type finder optical systems.

[1] An eyepiece optical system, comprising, in order from a side of animage being viewed,

a first lens having negative refracting power,

a second lens having positive refracting power,

a third lens having positive refracting power, and

a fourth lens having positive refracting power,

wherein the eyepiece optical system satisfies the following condition:

2.5≦f123/fA≦8  (4-1)

where f123 is a composite focal length of the first lens, the secondlens and the third lens, and

fA is a focal length of the eyepiece optical system.

[2] An eyepiece optical system, comprising, in order from a side of animage being viewed,

a first lens having negative refracting power,

a second lens having positive refracting power,

a third lens having positive refracting power, and

a fourth lens having positive refracting power,

wherein the eyepiece optical system satisfies the following condition:

1≦f4/fA≦2  (4-2)

where f4 is a focal length of the fourth lens, and

fA is a focal length of the eyepiece optical system.

[3] An eyepiece optical system, comprising, in order from a side of animage being viewed,

a first lens having negative refracting power,

a second lens having positive refracting power,

a third lens having positive refracting power, and

a fourth lens having positive refracting power,

wherein the eyepiece optical system satisfies the following condition:

0.02≦d4/fA≦0.2  (4-3)

where d4 is an axial thickness of the fourth lens, and

fA is a focal length of the eyepiece optical system.

[4] A relay type finder optical system, comprising:

a relay optical system operable to re-form a primary image formedthrough a taking optical system, and

an eyepiece optical system operable to view an image re-formed throughthe relay optical system, wherein:

the eyepiece optical system comprises, in order from a relay opticalsystem side,

a first lens having negative refracting power,

a second lens having positive refracting power,

a third lens having positive refracting power, and

a fourth lens having positive refracting power.

The fifth aspect of the invention includes the following relay typefinder optical systems.

[1] A relay type finder optical system, comprising:

a relay optical system operable to re-form a subject image at asecondary image-formation position wherein the subject image is aprimary image formed at a primary image-formation position through anobjective optical system, and

an eyepiece optical system operable to view an image re-formed throughthe relay optical system, wherein:

the relay type finder optical comprises at least three reflectingsurfaces located between the primary image-formation position and thesecondary image-formation position, and

the relay type finder optical system comprises a one-piece prism P1,wherein:

of the at least three reflecting surfaces, two reflecting surfaces lyingside by side on an optical axis in an optical path through the relayoptical system are internal reflecting surfaces of the prism P1, and twosuch reflecting surfaces are total-reflection surfaces.

[2] A relay type finder optical system, comprising:

a relay optical system operable to re-form a subject image at asecondary image-formation position wherein the subject image is aprimary image formed at a primary image-formation position through anobjective optical system, and

an eyepiece optical system operable to view an image re-formed throughthe relay optical system, wherein:

the relay type finder optical comprises at least three reflectingsurfaces located between the primary image-formation position and thesecondary image-formation position, and

the relay type finder optical system comprises a one-piece prism P1,wherein:

of the at least three reflecting surfaces, two reflecting surfaces lyingside by side on an optical axis in an optical path through the relayoptical system are internal reflecting surfaces of the prism P1, and anangle of incidence of an optical axis on two such internal reflectingsurfaces is greater than 45°.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical path diagram for one exemplary finder opticalsystem to which Embodiments 1-5 according to the first aspect of theinvention are applied.

FIG. 2 is illustrative of one exemplary practical layout of the finderoptical system of FIG. 1.

FIG. 3 is illustrative of a modification to that layout.

FIG. 4 is illustrative of the lenses to be shifted for anti-shake in thefinder optical system of Embodiment 1.

FIG. 5 is illustrative of the lenses to be shifted for anti-shake in thefinder optical system of Embodiment 2.

FIG. 6 is illustrative of the lenses to be shifted for anti-shake in thefinder optical system of Embodiment 3.

FIG. 7 is illustrative of the lenses to be shifted for anti-shake in thefinder optical system of Embodiment 4.

FIG. 8 is illustrative of the lenses to be shifted for anti-shake in thefinder optical system of Embodiment 5.

FIG. 9 is a collection of transverse aberration diagrams in a blur-freestate for the finder optical system of FIG. 1.

FIG. 10 is a collection of transverse aberration diagrams afteranti-shake operation by Embodiment 1.

FIG. 11 is a collection of transverse aberration diagrams afteranti-shake operation by Embodiment 2.

FIG. 12 is a collection of transverse aberration diagrams afteranti-shake operation by Embodiment 3.

FIG. 13 is a collection of transverse aberration diagrams afteranti-shake operation by Embodiment 4.

FIG. 14 is a collection of transverse aberration diagrams afteranti-shake operation by Embodiment 5.

FIG. 15 is illustrative in section of the construction of a single-lensreflex camera that incorporates the finder optical system of oneembodiment according to the second aspect of the invention.

FIG. 16 is illustrative of in what state the single-lens reflex cameraof FIG. 15 is during taking operation.

FIG. 17 is a more schematic representation of FIG. 15.

FIG. 18 is illustrative in schematic of one embodiment wherein a prismgroup having a half-silvered mirror surface is used in plane of thequick return mirror in FIGS. 15-17.

FIG. 19 is illustrative of a modification to the construction of FIG. 15wherein two plane mirrors are used for the prism in FIG. 15.

FIG. 20 is illustrative of a modification to the construction of thesingle-lens reflex camera of FIG. 15, wherein the prism is removed andthe angle of the quick return mirror with respect to the optical axis isan acute angle.

FIG. 21 is a more schematic representation of FIG. 20.

FIG. 22 is illustrative of in section of the finder optical system ofNumerical Embodiment 1 according to the second aspect of the invention,as taken apart along its optical axis.

FIG. 23 is illustrative of in section of the finder optical system ofNumerical Embodiment 2 according to the second aspect of the invention,as taken apart along its optical axis.

FIG. 24 is a collection of aberration diagrams for Numerical Embodiment1.

FIG. 25 is a collection of aberration diagrams for Numerical Embodiment2.

FIG. 26 is illustrative in section of the finder optical systemaccording to one embodiment of the third aspect of the invention, astaken apart along its optical axis.

FIG. 27 is illustrative in section of the finder optical system ofEmbodiment 2 according to the third aspect of the invention, as takenapart along its optical axis.

FIG. 28 is illustrative in section of the construction of a single-lensreflex camera that incorporates the finder optical system of Embodiment1.

FIG. 29 is illustrative of in what state the single-lens reflex cameraof FIG. 28 is during taking operation.

FIG. 30 is illustrative in section of the finder optical systemaccording to one embodiment of the fourth aspect of the invention, astaken apart along its optical axis.

FIG. 31 is illustrative in section of the finder optical system ofEmbodiment 2 according to the fourth aspect of the invention, as takenapart along its optical axis.

FIG. 32 is a collection of aberration diagrams for only the eyepieceoptical system in Numerical Embodiment 1.

FIG. 33 is a collection of aberration diagrams for only the eyepieceoptical system in Numerical Embodiment 2.

FIG. 34 is illustrative in section of the construction of a single-lensreflex camera that incorporates the finder optical system of Embodiment1.

FIG. 35 is illustrative of in what state the single-lens reflex cameraof FIG. 34 is during taking operation.

FIG. 36 is illustrative in section of the construction of a single-lensreflex camera that incorporates the relay type finder optical systemaccording to one embodiment according to the fifth aspect of theinvention.

FIG. 37 is illustrative of in what state the single-lens reflex cameraof FIG. 36 is during taking operation.

FIG. 38 is illustrative in schematic of one embodiment wherein a prismgroup having a half-silvered mirror surface is used in plane of thequick return mirror in FIG. 36.

FIG. 39 is illustrative, as in FIG. 36, of a modification wherein theprism is allowed to have a prism function.

FIG. 40 is illustrative, as in FIG. 36, of an embodiment wherein theprism is located between the positive lens group and the secondaryimage-formation position.

FIG. 41 is illustrative in section of the finder optical system ofNumerical Embodiment 2, as taken apart along its optical axis.

FIG. 42 is illustrative in section of the finder optical system ofNumerical Embodiment 3, as taken apart along its optical axis.

FIG. 43 is a collection of aberration diagrams for Numerical Embodiment2.

FIG. 44 is a collection of aberration diagrams for Numerical Embodiment3.

FIG. 45 is illustrative of the construction of one prior art opticalsystem provided with an anti-shake function.

FIG. 46 is illustrative of the construction of another prior art opticalsystem provided with an anti-shake function.

FIG. 47 is illustrative of the construction of yet another prior artoptical system provided with an anti-shake function.

FIG. 48 is illustrative of the construction of a further prior artoptical system provided with an anti-shake function.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Possible arrangements of the optical system, finder optical system,relay type finder optical system, eyepiece optical system andsingle-lens reflex camera of the invention are now explained withreference to the 1^(st) to 5^(th) aspects of the invention in thisorder.

The first aspect of the invention is first explained.

The first optical system according to the first aspect of the inventioncomprises a taking optical system and a finder optical system,characterized in that:

the taking optical system comprises an optical path splitter means forsplitting an optical path into the finder optical system,

the finder optical system comprises a relay optical system for forming asubject image once, and

an optical subsystem that forms a part of the finder optical system ismoved or shifted in a plane vertical to the optical axis of the finderoptical system for anti-shake (correction of camera-shake).

The advantage of, and the requirement for, the above first opticalsystem arrangement is now explained. This optical system corresponds toEmbodiments 1-5 given later.

In this arrangement, the taking optical system also works as anobjective lens in the finder optical system; any objective lens for thefinder optical system is dispensed with, so that the optical system canbecome simple.

An image viewed through the finder optical system is stabilized by ananti-shake mechanism, and so there is no shake of the image beingviewed, which is a problem especially with telephotography or fledglingcamera operators.

The location in the finder optical system of the relay optical systemfor forming a subject image once ensures that the image is easilyviewable by increasing the magnification of the relay optical system,and helps increase the degree of flexibility in implementing thecamera-shake correction function.

For a single-lens reflex camera like one contemplated herein, anerection optical system must be located between a primaryimage-formation plane and an eyepiece optical system, and to take holdof the angle of field, the focal length of the eyepiece optical systemmust be shortened. As the focal length is reduced while the pupildiameter and eye point position are kept constant, however, it causesthe effective diameter of the erection optical system to become large,resulting in difficulty in offering a sensible tradeoff between gettinghold of the angle of field and size reductions. However, this problemcan be solved by the adoption of the secondary image-formation techniqueof re-forming the image through the relay optical system.

By the adoption of the shift technique where the optical subsystem thatforms a part of the finder optical system moves in the plane vertical tothe optical axis of the finder optical system, there is no need ofanother element such as a variable vertex angle prism for camera-shakecorrection (anti-shake) purposes, so that the construction of theoptical system can become simple.

The second optical system according to the first aspect of the inventionis characterized in that the optical subsystem that forms the relayoptical system in the first optical system is shifted for anti-shake.

The advantage of, and the requirement for, the above second opticalsystem arrangement is now explained. This optical system corresponds toEmbodiments 1-4 given later.

The relay optical system is operable to re-form the primary image formedthrough the taking optical system in the form of a secondary image. Whena field frame is placed on the secondary image-formation plane, anotheranti-shake function is needed for that field frame unless the imagebeing viewed is corrected for camera-shake (anti-shake) on the secondaryimage-formation plane. If the anti-shake shift function of the opticalsystem that forms the relay optical system is implemented, the imagebeing viewed is prevented from shaking on the secondary image-formationplane; even without any anti-shake function for the field frame, boththe image being viewed and the field frame can be corrected forcamera-shake (anti-shake).

More preferably in this case, the relay optical system should include anoptical system (of triplet construction) composed of a positive lens, anegative lens and a positive lens in this order, because even bettercorrection of aberrations can be implemented with simple construction.

The third optical system according to the first aspect of the inventionis characterized in that the relay optical system in the first opticalsystem includes a field lens, and the anti-shake optical subsystem doesnot include that field lens.

The advantage of, and the requirement for, the above third opticalsystem arrangement is now explained.

If the relay optical system includes a field lens and that field lenshas a primary role of pupil position control, the diameter of thecomponents of the relay optical system other than the field lens canthen be decreased, although that field lens has a large diameter. Thefield lens positioned facing the primary image-formation plane or thesecond image-formation plane has a large effective diameter; if thefield lens is included in the anti-shake group, the weight of theanti-shake group adds up. This will often render it impossible to drivethe anti-shake group or drive it at high speed.

However, if the anti-shake function is implemented by the opticalsubsystem having a small diameter except the field lens, the load on theactuator can be lessened.

In addition, the sensitivity of the field lens to decentration is so lowthat the anti-shake function can be implemented by the shifting of theoptical subsystem except that field lens; even when relativemisalignments are caused by anti-shake operation between the anti-shakegroup and the field lens, the level of underperformance will remainlimited after anti-shake operation.

Because of being free from the field lens, the anti-shake group can becomposed of fewer lenses.

In this connection, the inclusion of the field lens on the secondaryimage-formation plane side of the relay optical system also provides thefollowing advantages. As the exit pupil of the relay optical system isformed on the image side with respect to the secondary image-formationplane, it allows for a decrease in the diameter of the eyepiece opticalsystem. This advantage grows more if the field lens primarily operableto control the pupil position is used for the lens on the secondaryimage-formation plane side of the relay optical system. The field lenshere should preferably be composed of one or two lenses.

The fourth optical system according to the first aspect of the inventionis characterized in that the anti-shake optical subsystem is includedbetween the pupil of the relay optical system and the field lens.

The advantage of, and the requirement for, the above fourth opticalsystem arrangement is now explained. This optical system corresponds toEmbodiments 3 and 4 given later.

The effective diameter of the optical system near the pupil is small,and so the effective diameter of the optical system between the pupiland the field lens is relatively small, too. If the anti-shake functionis implemented by the optical system between the pupil of relativelysmall size and the field lens, the weight of the anti-shake groupbecomes light enough to lessen loads on the anti-shake group drivingactuator.

The fifth optical system according to the first aspect of the inventionis characterized in that the relay optical system in the second opticalsystem comprises, in order from the primary image-formation plane side,a group of positive refracting power, a group of negative refractingpower and a group of positive refracting power, and the anti-shakeoptical subsystem is included in the last group of positive refractingpower.

The advantage of, and the requirement for, the above fifth opticalsystem arrangement is now explained. This optical system corresponds toEmbodiments 3 and 4 given later.

As the relay optical system is of the triplet construction comprising agroup of positive refracting power, a group of negative refracting powerand a group of positive refracting power in this order, it allows forcorrection of aberrations with simple construction. A high transversemagnification of the lens included in the last group of positiverefracting power contributes to an increased anti-shake sensitivity.

The sixth optical system according to the first aspect of the inventionis characterized in that the first optical system satisfies thefollowing condition:

0.8≦|Δ′/Δ|≦4  (1-1)

where:

Δ is the amount of shift of the moving optical subsystem, and

Δ′ is the amount of correction of an image position at the secondaryimage-formation plane.

The advantage of, and the requirement for, the above sixth opticalsystem arrangement is now explained. This optical system corresponds toEmbodiments 1-4 given later.

The satisfaction of condition (1-1) ensures that the anti-shakesensitivity (amount of image-shake correction/amount of shift of theanti-shake group) becomes proper. As the upper limit of 4 to thiscondition is exceeded, it renders tweak of the amount of shift of theanti-shake group difficult due to too high a sensitivity. As the lowerlimit of 0.8 is not reached, it causes large loads to be applied on theactuator, because too low a sensitivity renders the amount of shift ofthe anti-shake group large.

More preferably,

1≦|Δ′/Δ|≦3  (1-1)′

The above advantage grows more.

Even more preferably,

1.2≦|Δ′/Δ|≦2.2  (1-1)″

The above advantage grows much more.

The seventh optical system according to the first aspect of theinvention is characterized in that the second optical system satisfiesthe following condition:

0.5≦|f _(t) /f _(s)|≦8  (1-2)

where:

f_(t) is the focal length of the relay lens system, and

f_(s) is the focal length of the optical subsystem that shifts foranti-shake.

The advantage of, and the requirement for, the above seventh opticalsystem arrangement is now explained. This optical system corresponds toEmbodiments 1-4 given later.

The satisfaction of condition (1-2) ensures that a sensible tradeoff canbe made between anti-shake sensitivity and performance after anti-shake.If the upper limit of 8 to this condition is exceeded, the power of theanti-shake group will become too strong, resulting in considerableunderperformance after anti-shake. Falling short of the lower limit of0.5 will cause the power of the anti-shake group to become too weak,resulting in poor anti-shake sensitivity.

The eighth optical system according to the first aspect of the inventionis characterized in that the first optical system satisfies thefollowing condition:

−3≦β_(a)≦−0.5  (1-3)

where β_(a) is the transverse magnification of the optical subsystemthat moves for anti-shake.

The advantage of, and the requirement for, the above eighth opticalsystem arrangement is now explained. This optical system corresponds toEmbodiments 1-4 given later.

When a part of the optical system is shifted for anti-shake, thefollowing relation is satisfied:

Δ′=Δ(1−β_(a))β_(b)

where:

Δ is the amount of shift of the moving optical subsystem,

Δ′ is the amount of correction of an image position at the secondaryimage-formation plane,

β_(a) is the transverse magnification of the optical subsystem thatmoves for anti-shake, and

β_(b) is the transverse magnification of the relay lens group on theimage side with respect to the optical subsystem that moves foranti-shake.

The satisfaction of condition (1-3) ensures that the anti-shakesensitivity (amount of image-shake correction/amount of shift of theanti-shake group) becomes proper. As the lower limit of −3 to condition(1-3) is not reached, it renders tweak of the amount of shift of theanti-shake group difficult due to too high a sensitivity. As the upperlimit of −0.5 is exceeded, it causes large loads to be applied on theactuator, because too low a sensitivity renders the amount of shift ofthe anti-shake group large.

More preferably,

−2.7≦β_(a)≦−0.6  (1-3)′

The above advantage grows more.

Even more preferably,

−2.3≦β_(a)≦−0.7  (1-3)″

The above advantage grows much more.

According to the above first aspect of the invention, the anti-shakeoptical system that is reduced in terms of performance deteriorationafter anti-shake and has proper anti-shake sensitivity can be used tocorrect the finder optical system for camera-shake.

Reference is now made to the second aspect of the invention.

The first finder optical system according to the second aspect of theinvention comprises:

a relay optical system operable to re-form a subject image at asecondary image-formation position wherein the subject image is aprimary image formed at a primary image-formation position through anobjective optical system, and an eyepiece optical system operable toview an image re-formed through the relay optical system, characterizedin that:

the finder optical system comprises at least four reflecting surfacesincluding an F3 reflecting surface, an F2 reflecting surface, an F1reflecting surface and an R1 reflecting surface located between theprimary image-formation position and the secondary image-formationposition and in an optical path order from a primary image-formationposition side to a secondary image-formation position side,

the finder optical system has an optical axis reflected at each of thereflecting surfaces,

at least one positive lens is located at least between the F1 reflectingsurface and the R1 reflecting surface, and

when a direction of a light ray traveling on an optical axis is taken asan optical axis direction and a component parallel with an optical axisdirection exiting the eyepiece optical system is taken as a transversecomponent,

a transverse component in an optical axis direction exiting from the F2reflecting surface is opposite to an optical axis direction exiting theeyepiece optical system, and

a transverse component in an optical axis direction exiting from the R1reflecting surface is opposite to the optical axis direction exiting theeyepiece optical system. The second finder optical system according tothe second aspect of the invention comprises a relay optical systemoperable to re-form a subject image at a secondary image-formationposition wherein the subject image is a primary image formed at aprimary image-formation position through an objective optical system,and an eyepiece optical system operable to view an image re-formedthrough the relay optical system, characterized in that:

the finder optical system comprises at least four reflecting surfacesincluding an F3 reflecting surface, an F2 reflecting surface, an F1reflecting surface and an R1 reflecting surface located between theprimary image-formation position and the secondary image-formationposition and in an optical path order from a primary image-formationposition side to a secondary image-formation position side,

the finder optical system has an optical axis reflected at each of thereflecting surfaces,

at least one positive lens is located at least between the F1 reflectingsurface and the R1 reflecting surface, and

when a direction of a light ray traveling on an optical axis is taken asan optical axis direction and a component parallel with an optical axisdirection exiting the eyepiece optical system is taken as a transversecomponent,

a transverse component in an optical axis direction incident on the F1reflecting surface is opposite to an optical axis direction exiting theeyepiece optical system, and

a transverse component in an optical axis direction exiting from the R1reflecting surface is opposite to an optical axis direction exiting theeyepiece optical system.

The third finder optical system according to the second aspect of theinvention comprises a relay optical system operable to re-form a subjectimage at a secondary image-formation position wherein the subject imageis a primary image formed at a primary image-formation position throughan objective optical system, and an eyepiece optical system operable toview an image re-formed through the relay optical system, characterizedin that:

the finder optical system comprises at least four reflecting surfacesincluding an F3 reflecting surface, an F2 reflecting surface, an F1reflecting surface and an R1 reflecting surface located between theprimary image-formation position and the secondary image-formationposition and in an optical path order from a primary image-formationposition side to a secondary image-formation position side, and at leastone positive lens located between the F1 reflecting surface and the R1reflecting surface,

the finder optical system has an optical axis reflected at each of thereflecting surfaces,

-   -   at least one positive lens is located between the F1 reflecting        surface and the R1 reflecting surface,    -   an optical axis is acutely reflected at the F1 reflecting        surface and acutely reflected at the R1 reflecting surface, and

an angle that the F1 reflecting surface subtends the R1 reflectingsurface is an acute angle, provided that when at least one of the F1reflecting surface and the R1 reflecting surface is a curved reflectingmirror, the subtending angle is an angle that tangent planes subtendeach other at a position of incidence of an optical axis.

The advantages of, and the requirements for, the above 1st to 3^(rd)optical system arrangements are now explained.

To make the finder optical system compact in its height direction (withrespect to the optical axis of the objective optical system), the lensesin the relay optical system should preferably be laid out such that theydo not line up in the vertical direction to the objective opticalsystem. To make the objective optical system compact in its optical axisdirection, the relay optical system should preferably be configured suchthat the optical axis of its main part is not on the same straight lineas the optical axis of the eyepiece optical system.

On the other hand, the relay optical system must have reasonable spacesbetween the primary image-formation position and its principal pointpositions and between its principal point positions and the secondaryimage-formation position so as to implement its own function.

The satisfaction of the respective requirements for the 1^(st) to 3^(rd)finder optical systems according to the second aspect of the inventionas described above ensures that the lens element in the relay opticalsystem has the F1 reflecting surface and R1 reflecting surface beforeand after it, and that the lens element-receiving space can efficientlyand easily be secured between these reflecting surfaces. In addition, itis possible to achieve a relay optical system that is compact in bothits height direction and the optical axis direction of the objectiveoptical system.

Note here that the principal point positions of the relay optical systemare preferably found between the F1 reflecting surface and the R1reflecting surface.

The fourth finder optical system according to the second aspect of theinvention is characterized in that the first finder optical systemsatisfies the following condition (2-1) with respect to an angle α_(f)at which an optical axis direction exiting from the F2 reflectingsurface subtends an optical axis direction exiting from the eyepieceoptical system, provided however that when extensions of the above twooptical axis directions do not intersect, that angle α_(f) could begiven by an angle made upon projection of two such extensions in adirection of a straight line of connecting together their closestportions.

92°≦α_(f)≦135°  (2-1)

The fifth finder optical system according to the second aspect of theinvention is characterized in that the fourth finder optical systemsatisfies the following condition (2-1)′:

97°≦α_(f)≦105°  (2-1)

The advantages of, and the requirements for, the above 4^(th) and 5^(th)optical system arrangements are now explained.

Being shy of the lower limit of 92° to condition (2-1) will render itdifficult to get efficient hold of relay optical system space. If theupper limit of 1350 to condition (2-1) is exceeded, a lot more overlapswill be caused by the bending of a light beam, rendering the wholefinder system too bulky to figure out a good layout for it.

More preferably, the lower limit and the upper limit should be set 97°and 105°, respectively.

The sixth finder optical system according to the second aspect of theinvention is characterized in that the second finder optical systemsatisfies the following condition (2-2) with respect to an angle α_(f)′at which an optical axis direction incident on the F1 reflecting surfacesubtends an optical axis direction exiting from the eyepiece opticalsystem, provided that when extensions of the above two optical axisdirections do not intersect, that angle α_(f)′ could be given by anangle made upon projection of two such optical axis extensions in adirection of a straight line of connecting together their closestportions.

92°≦α_(f)′≦135°  (2-2)

The seventh finder optical system according to the second aspect of theinvention is characterized in that the sixth finder optical systemsatisfies the following condition (2-2)′:

92°≦α_(f)′≦105°  (2-2)′

The advantages of, and the requirements for, the above 6^(th) and 7^(th)optical system arrangements are now explained.

Being shy of the lower limit of 92° to condition (2-2) will render itdifficult to get efficient hold of relay optical system space. If theupper limit of 135° to condition (2-2) is exceeded, a lot more overlapswill be caused by the bending of a light beam, rendering the wholefinder system too bulky to figure out a good layout for it.

More preferably, the lower limit and the upper limit should be set 97°and 105°, respectively.

The eighth finder optical system according to the second aspect of theinvention is characterized in that the third finder optical systemsatisfies the following condition (2-3) with respect to the angle ofreflection, θ_(f), of an optical axis at the F1 reflecting surface.

45°<θ_(f)<88°  (2-3)

Note here that the angle of reflection, θ_(f), is an angle that theoptical axes incident on and reflected at the F1 reflecting surfacemake.

The ninth finder optical system according to the second aspect of theinvention is characterized in that the eighth finder optical systemsatisfies the following condition (2-3)′.

75°<θ_(f)<83°  (2-3)′

Being shy of the lower limit of 45° to condition (2-3) will render itdifficult to get efficient hold of relay optical system space. If theupper limit of 88° to condition (2-3) is exceeded, a lot more overlapswill be caused by the bending of a light beam, rendering the wholefinder system too bulky to figure out a good layout for it.

More preferably, the lower limit and the upper limit should be set 75°and 83°, respectively.

The 10^(th) finder optical system according to the second aspect of theinvention is characterized in that any one of the 3^(rd) and the8^(th)-9^(th) finder optical systems satisfies the following condition(2-4) with respect to the angle of reflection, or, of an optical axis atthe R1 reflecting surface.

45°<θ_(r)<88°  (2-4)

Note here that the angle of reflection, θ_(r), is an angle that theoptical axes incident on and reflected at the R1 reflecting surfacemake.

The 11^(th) finder optical system according to the second aspect of theinvention is characterized in that the 10^(th) finder optical systemsatisfies the following condition (2-4)′.

75°<θ_(r)<83°  (2-4)′

The advantages of, and the requirements for, the above 10^(th) and11^(th) finder optical systems are now explained.

Being shy of the lower limit of 45° to condition (2-4) will render itdifficult to get efficient hold of relay optical system space. If theupper limit of 88° to condition (2-4) is exceeded, a lot more overlapswill be caused by the bending of a light beam, rendering the wholefinder system too bulky to figure out a good layout for it.

More preferably, the lower limit and the upper limit should be set 75°and 83°, respectively.

The 12^(th) finder optical system according to the second aspect of theinvention is characterized in that any one of the 3^(rd) and the8^(th)-10^(th) finder optical systems satisfies the following condition(2-5) with respect to an angle θ_(m) at which the F1 reflecting surfacesubtends the R1 reflecting surface.

45°<θ_(m)<88°  (2-5)

It is noted, however, that when at least one of the F1 reflectingsurface and the R1 reflecting surface is a curved reflecting mirror,that angle θ_(m) could be given by an angle that tangent planes subtendeach other at a position of incidence of an optical axis.

The 13^(th) finder optical system according to the second aspect of theinvention is characterized in that the 12^(th) finder optical systemsatisfies the following condition (2-5)′.

75°<θ_(m)<83°  (2-5)

The advantages of, and the requirements for, the above 12^(th) and13^(th) finder optical systems are now explained.

Being shy of the lower limit of 45° to condition (2-5) will render itdifficult to get efficient hold of relay optical system space. If theupper limit of 88° to condition (2-5) is exceeded, a lot more overlapswill be caused by the bending of a light beam, rendering the wholefinder system too bulky to figure out a good layout for it.

More preferably, the lower limit and the upper limit should be set 75°and 83°, respectively.

The 14^(th) finder optical system according to the second aspect of theinvention is characterized in that in any one of the 1^(st) and the2^(nd) finder optical system, an optical axis just upon exiting from theR1 reflecting surface subtends an optical axis exiting from the eyepieceoptical system at an acute angle α_(r) that satisfies the followingcondition (2-6), provided however that when extensions of the above twooptical axis directions do not intersect, that angle α_(f) could begiven by an angle made upon projection of two such extensions in adirection of a straight line of connecting together their closestportions.

45°≦α_(r)≦88°  (2-6)

The 15^(th) finder optical system according to the second aspect of theinvention is characterized in that the 14^(th) finder optical systemsatisfies the following condition (2-6)′.

60°≦α_(r)≦80°  (2-6)′

The advantages of, and the requirements for, the above 14^(th) and15^(th) finder optical systems are now explained.

If the lower limit of 45° to condition (2-6) is not reached, a lot moreoverlaps will be caused by the bending of a light beam, rendering thewhole finder system too bulky to figure out a good layout for it.Exceeding the upper limit of 88° to condition (2-6) will render itdifficult to get efficient hold of relay optical system space.

More preferably, the lower limit and the upper limit should be set 60°and 80°, respectively.

The 16^(th) finder optical system according to the second aspect of theinvention is characterized in that the direction of an optical axisincident on the F1 reflecting surface is away from the primaryimage-formation position and the direction of an optical axis exitingfrom the R1 reflecting surface is toward the primary image-formationposition.

The advantage of, and the requirement for, the above 16^(th) finderoptical system arrangement is now explained.

With such arrangement, it is possible to get hold of the space betweenthe F1 reflecting surface and the R1 reflecting surface, and figure outa compact layout for the whole finder optical system including the relayoptical system and the eyepiece optical system.

In particular, the location of the F1 reflecting surface on a subjectside with respect to the primary image-formation position and thelocation of the R1 reflecting surface on a viewer side with the respectto the primary image-formation position are more preferable for thatcompact layout.

The 17^(th) finder optical system according to the second aspect of theinvention is characterized in that in any one of the 1^(st) to the16^(th) finder optical system, at least one positive lens is locatedbetween the F1 reflecting surface and the R1 reflecting surface, atleast one negative lens is located on the side of the positive lens thatfaces the R1 reflecting surface, and at least one positive lens islocated on the side of the negative lens that faces the R1 reflectingsurface.

The advantage of, and the requirement for, the above 17^(th) finderoptical system arrangement is now explained.

If at least the positive lens, the negative lens and the positive lensare located in relatively proximate relations without any reflectingsurface interposed between the lenses, it is then possible to configurea compact yet good-performance relay optical system.

The 18^(th) finder optical system according to the second aspect of theinvention is characterized in that in any one of the 1^(st) to the17^(th) finder optical system, the position of incidence of an opticalaxis on the F1 reflecting surface with respect to the position ofincidence of an optical axis on the F3 reflecting surface is in adirection away from the direction of an optical axis exiting theeyepiece optical system.

The advantage of, and the requirement for, the above 18^(th) finderoptical system arrangement is now explained.

With that arrangement, it is possible to get hold of a space capable oflaying out the relay optical system on a straight line substantiallyparallel with the optical axis exiting the eyepiece optical system.

The 19^(th) finder optical system according to the second aspect of theinvention is characterized in that optical function surfaces locatedbetween the F3 reflecting surface and the F1 reflecting surfaces are allin plane form.

The advantage of the above 19^(th) finder optical system arrangement isnow explained.

That arrangement is easy to assemble.

The 20^(th) finder optical system according to the second aspect of theinvention is characterized in that in any one of the 1st to the 19^(th)finder optical system, an R2 reflecting surface is located between theR1 reflecting surface and the secondary image-formation position, and anoptical axis exiting from the secondary image-formation position and anoptical axis incident on the eyepiece optical system lie on the samestraight line.

The advantage of, and the requirement for, the about 20^(th) finderoptical system arrangement is now explained.

With that arrangement, it is possible to get hold of a space between thesecondary image-formation position and the principal point positions ofthe relay optical system, and it is easy to figure out a layout wherethe secondary image-formation position comes close to the eyepieceoptical system. This works for configuring a relay type finder systemhaving improved performance at a high finder magnification.

The 21^(st) finder optical system according to the second aspect of theinvention is characterized in that any one of the 1^(st) to the 20^(th)finder optical system, an R2 reflecting surface is located between theR1 reflecting surface and the secondary image-formation position, and atleast one positive lens is located between the R1 reflecting surface andthe R2 reflecting surface.

The advantage of, and the requirement for, the above 21^(st) finderoptical system arrangement is now explained.

If a lens having the image-formation function of the relay opticalsystem and a pupil transmission function from secondary image-formationto the viewer's pupil is located at a position reasonably away from boththe principal points of the relay optical system located between the F1reflecting surface and the R1 reflecting surface and the secondaryimage-formation plane, it is then easy to configure a compact,easy-to-view, high-performance finder.

The 22^(nd) finder optical system according to the second aspect of theinvention is characterized in that the number of reflecting surfacesbetween the primary image-formation position and the secondaryimage-formation position is only five.

The advantage of, and the requirement for, the above 22^(nd) finderoptical system arrangement is now explained.

As there are six or more reflecting surfaces between the primaryimage-formation position and the secondary image-formation position,light quantity losses due to them grow large. If the number ofreflecting surfaces is five, it is then possible to get hold of thedegree of flexibility in determining what layout is used.

One single-lens reflex camera according to the second aspect of theinvention is characterized by comprising a light beam splitter meansoperable to split light beams entered through an objective opticalsystem into a light beam incident on an image pickup device and a lightbeam incident on a finder optical system operable to bend the light beamto view a subject image, a finder optical system operable to bend thelight beam coming from the objective optical system to view a subjectimage, and a focal plane plate placed at a primary image-formationposition on a surface optically equivalent to the image pickup device toform a subject image, wherein the finder optical system is any one ofthe 1^(st) to the 22^(nd) finder optical system as described above.

The advantage of, and the requirement for, the above single-lens reflexcamera arrangement according to the second aspect of the invention isnow explained.

The above finder arrangement is mounted on a single-lens reflex camera.Note here that the light beam splitter means could be a light quantitydivision means relying on a half-silvered mirror or the like, or a timedivision means relying on a quick return mirror or the like.

Another single-lens reflex camera according to the second aspect of theinvention is characterized by comprising a light beam splitter meansoperable to split light beams entered through an objective opticalsystem into a light beam incident on an image pickup device and a lightbeam incident on a finder optical system operable to bend the light beamto view a subject image, and a finder optical system operable to bendthe light beam coming from the objective optical system to view asubject image, wherein:

the finder optical system comprises:

a focal plane plate placed at a primary image-formation position set ona surface optically equivalent to the image pickup device,

a relay optical system operable to re-form a subject image at asecondary image-formation position wherein the subject image is aprimary image formed at the primary image-formation position, and

an eyepiece optical system operable to view an image re-formed via therelay optical system, and

the finder optical system comprises, in order from an optical path orderfrom the primary image-formation position side, an F1 reflectingsurface, an R1 reflecting surface and an R2 reflecting surface, wherein:

the finder optical system has an optical axis reflected at each of thereflecting surfaces,

at least one positive lens is located at least between the F1 reflectingsurface and the R1 reflecting surface,

an optical axis is acutely reflected at the light beam splitter means,an optical axis is acutely reflected at the F1 reflecting surface, andan optical axis is acutely reflected at the R1 reflecting surface,

an angle that the F1 reflecting surface subtends the R1 reflectingsurface is an acute angle, provided however that when at least one ofthe F1 reflecting surface and the R1 reflecting surface is a curvedreflecting mirror, the angle that the F1 reflecting surface subtends theR1 reflecting surface could be given by an angle that tangent planessubtend each other at a position of incidence of an optical axis, and

an optical axis of the eyepiece optical system is substantially parallelwith an optical axis of the objective optical system.

The advantage of, and the requirement for, the above single-lens reflexcamera arrangement is now explained.

With that layout, the optical path through the finder optical system canbe turned back with fewer reflecting surfaces. Thus, a compactsingle-lens reflex camera can be configured albeit having a secondaryimage-formation type finder. It is also possible to configure asingle-lens reflex camera having a high finder magnification.

In accordance with the above second aspect of the invention, it ispossible to achieve a relay type finder optical system whereinreflecting surfaces are located before and after a lens element in arelay optical system so that the finder optical system can be madecompact in both its height direction and the optical axis direction ofan objective optical system. It is thus possible to provide a compactrelay type finder optical system well compatible even with a single-lensreflex camera using a relatively small image pickup device, and asingle-lens reflex camera using it.

Next, reference is made to the third aspect of the invention.

The first finder optical system according to the third aspect of theinvention comprises a relay optical system operable to re-form a subjectimage at a secondary image-formation position wherein the subject imageis a primary image formed at a primary image-formation position throughan objective optical system, and an eyepiece optical system operable toview an image re-formed through the relay optical system, characterizedin that:

the relay optical system comprises an FP lens group of positiverefracting power, an N lens group of negative refracting power and an RPlens group of positive refracting power, wherein:

the FP lens group of positive refracting power consists of a first lenshaving positive refracting power, and a second lens located on a side ofthe first lens facing the eyepiece optical system and having positiverefracting power,

the N lens group of negative refracting power consists of a third lensgroup located on an eyepiece optical system side with respect to the Nlens group and having negative refracting power, and

the RP lens group of positive refracting power consists of a fourth lenslocated on an eyepiece optical system side with respect to the N lensgroup and having negative refracting power and a fifth lens located on aside of the fourth lens facing an eyepiece optical system and havingpositive refracting power, and

the relay optical system satisfies the following condition with respectto a composite focal length, flr, of the EP lens group, the N lens groupand the RP lens group, and an axial distance, dl, between the primaryimage-formation position and a side of the first lens facing a primaryimage-formation position:

0.3<dl/flr<3  (3-1)

The advantage of, and the requirement for, the above first finderoptical system arrangement according to the third aspect of theinvention is now explained.

In the relay lens of the invention, the positive FP lens group, thenegative N lens group and the positive RP lens group are located in thisorder for the purpose of implementing functions of re-forming an imageand making correction of aberrations. The location of these lens groupsimproves on refracting power symmetry, working for correction ofaberrations.

In addition, more satisfactory correction of aberrations is achievableif the positive refracting power of the positive FP lens group isallocated to two positive lenses such that the necessary power isobtainable with lens surfaces having a slack curvature.

For correction of chromatic aberrations, the positive RP lens group iscomposed of the negative lens and the positive lens, in order from theprimary image-formation plane side, two in all.

Being short of the lower limit of 0.3 to the above condition (3-1) isnot preferable, because the first lens comes too close to the primaryimage-formation plane, and the power of the first lens contributes lessto the re-formation of images. With this, increased loads are applied onthe re-formation of images by the second lens, and the fifth lens,rendering spherical aberrations, etc. likely to occur. As the upperlimit of 3 to condition (3-1) is exceeded, it renders the whole relayoptical system length likely to become long. When the relay opticalsystem is configured while its whole length remains short, it isdifficult to implement correction of aberrations with a small number oflenses. In other words, a lot more lenses must be used for that purpose,working against compactness and a paraxial arrangement.

Regarding the above condition (3-1), the lower limit should preferablybe set at 0.8, especially 1.2, and the upper limit should preferably beset at 2.4, especially 1.8.

The second finder optical system according to the third aspect of theinvention comprises a relay optical system operable to re-form a subjectimage at a secondary image-formation position wherein the subject imageis a primary image formed at a primary image-formation position throughan objective optical system, and an eyepiece optical system operable toview an image re-formed through the relay optical system, characterizedin that:

the relay optical system comprises an FP lens group of positiverefracting power, an N lens group located on a side of the FP lens groupfacing the eyepiece optical system and having negative refracting power,and an RP lens group located on a side of the N lens group facing theeyepiece optical system and having positive refracting power, wherein:

the RP lens group consists of, in order from a primary image-formationposition side, a negative lens and a positive lens, and

the relay optical system satisfies the following condition (3-2) withrespect to an axial distance, drf, from a surface located in, andnearest to a secondary image-formation position side of, the FP lensgroup to a surface located in the N lens group and on a secondaryimage-formation position side and an axial distance, drr, from a surfacelocated in the N lens group and on a secondary image-formation positionside to a surface located in, and nearest to a primary image-formationposition side of, the RP lens group:

0.2<drf/drr<0.8  (3-2)

The advantage of, and the requirement for, the above second finderoptical system is now explained.

In the relay lens of the invention, the positive FP lens group, thenegative N lens group and the positive RP lens group are located in thisorder for the purpose of implementing functions of re-forming an imageand making correction of aberration. The location of these lens groupsimproves on refracting power symmetry, working for correction ofaberrations.

Further, it is preferable for the RP lens to consist of a negative lensand a positive lens and for the relay optical system to satisfycondition (3-2), because the distance between the negative component inthe N lens group and the negative lens in the RP lens group can beproperly set, thereby implementing satisfactory correction oflongitudinal chromatic aberration and chromatic aberration ofmagnification.

Being shy of the lower limit of 0.2 to condition (3-2) is not preferablein view of compactness and a paraxial arrangement, because the N lensgroup is spaced too away from the RP lens group, resulting in anincrease in the total length of the relay optical system, and anincrease in the number of lenses used in it. Exceeding the upper limitof 0.8 renders it difficult to offer a balance against correction oflongitudinal chromatic aberration and off-axis chromatic aberration,because the negative lenses come too close to each other.

Preferably, the N lens group should be composed of one negative lens.This can make the relay optical system compact. Preferably, the FP lensgroup should be composed of two positive lenses. This enables thenecessary power to be obtained at lens surfaces having a slackcurvature, working for satisfactory correction of aberrations.

Regarding the above condition (3-2), the lower limit should preferablybe set at 0.3, especially 0.4, and the upper limit should be set at0.65, especially 0.55.

The third finder optical system according to the third aspect of theinvention comprises a relay optical system operable to re-form a subjectimage at a secondary image-formation position wherein the subject imageis a primary image formed at a primary image-formation position throughan objective optical system, and an eyepiece optical system operable toview an image re-formed through the relay optical system, characterizedin that:

the relay optical system comprises an FP lens group of positiverefracting power, an N lens group of negative refracting power and an RPlens group of positive refracting power, wherein:

the FP lens group of positive refracting power consists of a first lenshaving positive refracting power, and a second lens located on a side ofthe first lens facing the eyepiece optical system and having positiverefracting power,

the N lens group of negative refracting power consists of a third lensgroup located on an eyepiece optical system side with respect to the FPlens group and having negative refracting power, and

the RP lens group of positive refracting power is located on an eyepieceoptical system side with respect to the N lens group, and

the relay optical system satisfies the following conditions (3-1) and(3-3) with respect to a composite focal length, flr, of the EP lensgroup, the N lens group and the RP lens group, an axial distance, dl,between the primary image-formation position and a side of the firstlens facing the primary image-formation position, and an axial distance,ds, from a surface located in the RP lens group and on a secondaryimage-formation position side to the secondary image-formation position:

0.3<dl/flr<3  (3-1)

0.5<ds/flr<2  (3-3)

The advantage of, and the requirement for, the above third finderoptical system arrangement according to the third aspect of theinvention is now explained.

In the relay lens of the invention, the positive FP lens group, thenegative N lens group and the positive RP lens group are located in thisorder for the purpose of implementing functions of re-forming an imageand making correction of aberrations. The location of these lens groupsimproves on refracting power symmetry, working for correction ofaberrations.

In addition, more satisfactory correction of aberrations is achievableif the positive refracting power of the positive FP lens group isallocated to two positive lenses such that the necessary power isobtainable with lens surfaces having a slack curvature.

Being short of the lower limit of 0.3 to the above condition (3-1) isnot preferable, because the first lens comes too close to the primaryimage-formation plane, and the power of the first lens contributes lessto the re-formation of images. With this, increased loads are applied onthe re-formation of images by the second lens, and the positive lens inthe RP lens group, rendering spherical aberrations, etc. likely tooccur. As the upper limit of 3 to condition (3-1) is exceeded, itrenders the whole relay optical system length likely to become long.When the relay optical system is configured while its whole lengthremains short, it is difficult to implement correction of aberrationswith a small number of lenses. In other words, a lot more lenses must beused for that purpose, working against compactness and a paraxialarrangement.

Being short of the lower limit of 0.5 to the above condition (3-3) isnot preferable, because the RP lens groups comes too close to thesecondary image-formation plane, and the power of the RP lens groupcontributes less to the re-formation of images. With this, increasedloads are applied on the first lens and the second lens, renderingspherical aberrations likely to occur. Exceeding the upper limit of 3 isnot preferable in view of compactness and paraxial arrangement, becausethe whole relay optical system length becomes long with an increase inthe number of lenses used.

Regarding the above condition (3-1), the lower limit should preferablybe set at 0.8, especially 1.2, and the upper limit should preferably beset at 2.4, especially 1.8.

Regarding the above condition (3-3), the lower limit should preferablybe set at 0.7, especially 1.0, and the upper limit should preferably beset at 1.7, especially 1.5.

In the invention, the arrangement of the first finder optical system,the arrangement of the second finder optical system and the arrangementof the third finder optical system could be applied in combination oftwo or more.

Typically in this regard, the fourth finder optical system according tothe third aspect of the invention comprises a relay optical systemoperable to re-form a subject image at a secondary image-formationposition wherein the subject image is a primary image formed at aprimary image-formation position through an objective optical system,and an eyepiece optical system operable to view an image re-formedthrough the relay optical system, characterized in that:

the relay optical system comprises an FP lens group of positiverefracting power, an N lens group of negative refracting power and an RPlens group of positive refracting power, wherein:

the FP lens group of positive refracting power consists of a first lenshaving positive refracting power, and a second lens located on a side ofthe first lens facing the eyepiece optical system and having positiverefracting power,

the N lens group of negative refracting power consists of a third lensgroup located on an eyepiece optical system side with respect to the FPlens group and having negative refracting power, and

the RP lens group of positive refracting power consists of a fourth lenslocated on an eyepiece optical system side with respect to the N lensgroup and having negative refracting power, and a fifth lens located ona side of the fourth lens facing the eyepiece optical system and havingpositive refracting power, and

the relay optical system satisfies the following conditions (3-1), (3-2)and (3-3) with respect to a composite focal length, flr, of the EP lensgroup, the N lens group and the RP lens group, an axial distance, dl,between the primary image-formation position and a side of the firstlens facing the primary image-formation position, an axial distance,drf, from a surface located in, and nearest to a secondaryimage-formation position side of, the FP lens group to a surface locatedin the N lens group and on a secondary image-formation position side, anaxial distance, drr, from a surface located in the N lens group and on asecondary image-formation position side to a surface located in, andnearest to a primary image-formation position side of, the RP lens, andan axial distance, ds, from a surface located in the RP lens group andon a secondary image-formation position side to the secondaryimage-formation position:

0.3<dl/flr<3  (3-1)

0.2<drf/drr<0.8  (3-2)

0.5<ds/flr<2  (3-3)

The fifth finder optical system according to the third aspect of theinvention is characterized in that in any one of the 1^(st) to the4^(th) finder optical system, a relay optical system auxiliary lens islocated between the surface located in the positive RP lens group and onthe secondary image-formation position side and the secondaryimage-formation position in such a way as to satisfy condition (3-4):

0.25<dh/ds<0.75  (3-4)

where:

dh is an axial distance from the surface located in the RP lens groupand on the secondary image-formation position side to a side of therelay optical system auxiliary lens facing an RP lens group side, and

ds is an axial distance from the surface located in the RP lens groupand on the secondary image-formation position side to the secondaryimage-formation position.

The advantage of, and the requirement for, the above fifth finderoptical system arrangement is now explained.

As some distance is set between the RP lens group and the relay opticalsystem auxiliary lens, it allows an axial light beam to be so separatedfrom an off-axis light beam that effects on correction of off-axisaberrations can grow more. It also provides a backup to a condenserfunction and a pupil aberration correction function during the secondaryimage-formation operation. Being shy of the lower limit of 0.25 tocondition (3-4) will render the separation between the axial light beamand the off-axis light beam insufficient, and exceeding the upper limitof 0.75 will cause them to come too close to the image-formationpositions, rendering it difficult to obtain any effect on correction ofimage-formation aberrations. Preferably, the relay optical systemauxiliary lens should be a positive lens.

Regarding the above condition (3-4), the lower limit should preferablybe set at 0.3, especially 0.4, and the upper limit should preferably beset at 0.6, especially 0.5.

The sixth finder optical system according to the third aspect of theinvention is characterized in that in the fifth finder optical system,the relay optical system auxiliary lens has an aspheric surface appliedto at least one surface.

The advantage of, and the requirement for, the above sixth finderoptical system is now explained.

As the aspheric surface is applied to the relay optical system auxiliarylens, an easy-to-separate off-axis light beam is effectivelycontrollable, adding up to growing effects on correction of off-axisaberrations.

The seventh finder optical system according to the third aspect of theinvention is characterized in that in any one of the 1^(st) to the6^(th) finder optical system, the RP lens group is composed of acemented doublet wherein a negative lens and a positive lens arecemented together.

The advantage of, and the requirement for, the above 7^(th) finderoptical system arrangement is now explained.

Constructing the RP lens group of a cemented doublet helps decrease thesensitivity to decentration, and works for correction of chromaticaberrations. This also help prevent the occurrence of higher-orderaberrations with easy lens thickness control.

Any one of the above 1^(st) to 7^(th) finder optical systems could beused together with a focal plane plate located at the primaryimage-formation position into a single-lens reflex camera that enables asubject to be well viewed even with a small image pickup device.

In accordance with the third aspect of the invention as described above,it is possible to achieve a compact relay type finder optical systemthat is improved in refractive power symmetry and well corrected foraberrations. It is thus possible to provide a compact relay type finderoptical system well compatible with a single-lens reflex camera using arelatively small image pickup device and a single-lens reflex cameraincorporating it.

Next, reference is made to the fourth aspect of the invention.

The first embodiment of the fourth aspect of the invention is directedto an eyepiece optical system characterized by comprising, in order froma side of the image being viewed, a first lens having negativerefracting power, a second lens having positive refracting power, athird lens having positive refracting power and a fourth lens havingpositive refracting power, and satisfying the following condition:

2.5≦f123/fA≦8  (4-1)

where:

f123 is the composite focal length of the first lens, the second lensand the third lens, and

fA is the focal length of the whole eyepiece optical system.

The advantage of, and the requirement for, the above first embodiment isnow explained.

In this eyepiece optical system comprising the first to the fourth lens,there is a −+ sharing of power in order from the side of the image beingviewed, so that the principal points are located on the pupil side. Thisallows the space between the image being viewed and the eyepiece opticalsystem to become so narrow that the total length of the eyepiece opticalsystem including the position of the image being viewed can beminimized.

To improve on the performance of the eyepiece optical system whileenhancing this effect and beefing up its power, three positive lensesare located on the side of the negative lens facing the viewer todisperse the refraction of a light beam.

Condition (4-1) is provided to make the eyepiece optical system compact.As the lower limit of 2.5 to condition (4-1) is not reached, it causesthe negative power to become weak or the power of the positive lenses tobecome too strong, resulting in a shift of the principal point positionstoward the image side and contributing little to the decrease in thetotal length of the eyepiece optical system including the position ofthe image being viewed. This also works against correction of chromaticaberrations. As the upper limit of 8 is exceeded, on the other hand, itcauses the negative power to become too strong, resulting in an increasein the outside diameter of the lens. This also works against correctionof off-axis aberrations.

Regarding condition (4-1), the lower limit could be set at 3.5,especially 4.0, and the upper limit could be set at 6.5, especially 5.0.

The second embodiment of the fourth aspect of the invention is directedto an eyepiece optical system characterized by comprising, in order froma side of the image being viewed, a first lens having negativerefracting power, a second lens having positive refracting power, athird lens having positive refracting power and a fourth lens havingpositive refracting power, and satisfying the following condition:

1≦f4/fA≦2  (4-2)

where:

f4 is the focal length of the fourth lens, and

fA is the focal length of the eyepiece optical system.

The advantage of, and the requirement for, the above second embodimentis now explained.

In this eyepiece optical system comprising the first to the fourth lens,there is a −+ sharing of power in order from the image being viewed, sothat the principal points are located on the pupil side. This allows thespace between the image being viewed and the eyepiece optical system tobecome so narrow that the total length of the eyepiece optical systemincluding the position of the image being viewed can be minimized.

To improve on the performance of the eyepiece optical system whileenhancing this effect and beefing up its power, three positive lensesare located on the side of the negative lens facing the viewer todisperse the refraction of a light beam.

Condition (4-2) is provided to define the refracting power sharing ofthe fourth lens group. As the lower limit of 1 to condition (4-2) is notreached, a light beam passing through the third lens group becomesslender, and there is no or little separation of an axial light beamfrom an off-axis light beam, which renders correction of aberrationsdifficult. As the upper limit of 2 to condition (4-2) is exceeded, onthe other hand, the outside diameter of the lens for getting hold of aneye relief is likely to become large. In addition, the positive power ofthe fourth lens becomes weak or the refracting powers of the second andthird lenses become strong, working against correction of sphericalaberrations.

Regarding condition (4-2), the lower limit could be set at 1.4,especially 1.5, and the upper limit could be set at 1.9, especially 1.8.

The third embodiment of the fourth aspect of the invention is directedto an eyepiece optical system characterized by comprising, in order fromthe image being viewed, a first lens having negative refracting power, asecond lens having positive refracting power, a third lens havingpositive refracting power and a fourth lens having positive refractingpower, and satisfying the following condition:

0.02≦d4/fA≦0.2  (4-3)

where:

d4 is the axial thickness of the fourth lens, and

fA is the focal length of the eyepiece optical system.

The advantage of, and the requirement for, the above third embodiment isnow explained.

In this eyepiece optical system comprising the first to the fourth lens,there is a −+ sharing of power in order from the image being viewed, sothat the principal points are located on the pupil side. This allows thespace between the image being viewed and the eyepiece optical system tobecome so narrow that the total length of the eyepiece optical systemincluding the position of the image being viewed can be minimized.

To improve on the performance of the eyepiece optical system whileenhancing this effect and beefing up its power, three positive lensesare located on the side of the negative lens facing the viewer todisperse the refraction of a light beam.

Condition (4-3) is provided to define the thickness of the fourth lens.If the lower limit of 0.02 to this condition is not reached, it will bedifficult for the fourth lens to have sufficient power and therefracting powers of the second and third lenses will become strong,rendering correction of spherical aberrations difficult. As the upperlimit of 0.2 to condition (4-3) is exceeded, it causes a light beampassing through the third lens group to become slender with the resultthat there is no or little separation of an axial light beam from anoff-axis light beam, which renders correction of aberrations difficult.In addition, the total length of the eyepiece optical system becomeslong.

Regarding condition (4-3), the lower limit could be set at 0.05,especially 0.09, and the upper limit could be set at 0.17, especially0.15.

The fourth embodiment according to the fourth aspect of the invention ischaracterized in that in any one of the 1^(st) to the 3^(rd) embodiment,the image being viewed is an image or aerial image formed by animage-formation lens.

The advantage of, and the requirement for, the above fourth embodimentis now explained.

If the image being viewed is formed by the image-formation lens, theexit range of a light beam can then be defined by the exit pupil of theimage-formation lens or the like. This helps prevent the peripheralportion of an image guided into the viewer's eyeball from becoming darkand cut off unnecessary light with the image-formation lens before theunnecessary light—responsible for flares—is directed into the eyepieceoptical system. Thus, the outside diameter of the eyepiece opticalsystem can be made compact while holding back the occurrence of flares.

The fifth embodiment according to the fourth aspect of the invention isdirected to a relay type finder optical system comprising a relayoptical system operable to re-form a primary image formed through ataking optical system and an eyepiece optical system operable to view animage re-formed through the relay optical system, characterized in that:

the eyepiece optical system comprises, in order from the relay opticalsystem side, a first lens having negative refracting power, a secondlens having positive refracting power, a third lens having positiverefracting power and a fourth lens having positive refracting power.

The advantage of, and the requirement for, the above fifth embodiment isnow explained.

Even when the primary image-formation plane formed by the taking opticalsystem is small, the image formed through the taking optical system isre-formed through the relay optical system, whereby the space betweenthe image being viewed and the eyepiece optical system can be madenarrow. It is this possible to achieve a finder optical system with awide angle of field.

In addition, the eyepiece optical system is configured such that a −+sharing of power is provided as viewed from the relay optical system tolocate the principal points on the pupil side. This enables the spacebetween the re-formed image and the eyepiece optical system to be sonarrower that the total length of the eyepiece optical system includingthe image position can be more reduced.

To improve on the performance of the eyepiece optical system whileenhancing this effect and beefing up its power, and to get hold of aneye point while keeping the lens outside diameter compact, threepositive lenses are located on the side of the negative lens facing theviewer to disperse the refraction of a light beam.

The sixth embodiment according to the fourth aspect of the invention ischaracterized in that the fifth embodiment satisfies the followingcondition:

17 mm≦fA≦40 mm  (4-4)

where fA is the focal length of the eyepiece optical system.

The advantage of, and the requirement for, the above sixth embodiment isnow explained.

Condition (4-4) is provided to define the focal length of the wholeeyepiece optical system. If the lower limit of 17 mm to condition (4-4)is not reached, it will be difficult for the primary image-formationplane to have sufficient size while getting hold of an eye point, andaberrations will occur at the eyepiece optical system as well. If theupper limit of 40 mm to condition (4-4) is exceeded, on the other hand,it will be difficult to offer any sensible tradeoff between thesufficient angle of field and the compactness of the secondaryimage-formation plane.

Regarding condition (4-4), the lower limit could be set at 20 mm,especially 24 mm, and the upper limit could be set at 35 mm, especially30 mm.

The seventh embodiment according to the fourth aspect of the inventionis directed to a relay type finder optical system comprising a relayoptical system operable to re-form a primary image formed through ataking optical system and an eyepiece optical system operable to view animage re-formed through the relay optical system, characterized in that:

any one of the 1^(st) to the 4^(th) embodiment is used as the eyepieceoptical system.

The advantage of, and the requirement for, the above seventh embodimentis now explained.

If the eyepiece optical system according to any one of the 1^(st) to the4^(th) embodiment is used for the relay type finder optical system, theimage formed through the taking optical system is then re-formed by therelay optical system whereby the space between the image being viewedand the eyepiece optical system can be made narrow, even when theprimary image-formation plane formed through the taking optical systemis small. It is thus possible to achieve a finder optical system with awide angle of field.

The eighth embodiment according to the fourth aspect of the invention ischaracterize in that in the relay type finder optical system accordingto any one of the 5^(th) to the 7^(th) embodiment, the fourth lensremains fixed, and the first lens, the second lens and the third lensare operable to move together in the optical axis direction, therebyimplementing diopter control.

The advantage of, and the requirement for, the above 8^(th) embodimentis now explained.

If the first, the second and the third lens are operable to movetogether for diopter control of the eyepiece optical system, therefracting power of the moving group can then be held back to reducechanges of magnification and aberrations during the diopter control.

With this embodiment, the drive system is simpler in construction thanthat for driving the whole eyepiece optical system. In addition, alarger amount of movement is applied to the same diopter change ascompared with the integral movement of the whole eyepiece opticalsystem, making tweak easier.

Because the fourth lens remains fixed, it is possible for the fourthlens to have a cover glass function for the viewer side, therebydispensing with any cover glass.

In particular, this embodiment should preferably have satisfied theabove condition (4-1). This works for balancing aberration fluctuationsagainst the amount of movement during the diopter control.

The ninth embodiment according to the fourth aspect of the invention ischaracterized in that in the relay type finder optical system accordingto any one of the 1^(st) to the 4^(th) embodiment, the fourth lensremains fixed, and the first lens, the second lens and the third lensare operable to move together in the optical axis direction, therebyimplementing diopter control.

The advantage of, and the requirement for, the above 9^(th) embodimentis the same as is the case with the 8^(th) embodiment.

The 10^(th) embodiment according to the fourth aspect of the inventionis characterized in that in any one of the 1^(st) to 4^(th) embodimentand the 9^(th) embodiment, the first lens and the second lens arecemented together into a cemented doublet.

The 11^(th) embodiment according to the fourth aspect of the inventionis characterized in that in any one of the 5^(th) to the 8^(th)embodiment, the first lens and the second lens are cemented togetherinto a cemented doublet.

The advantages of, and the requirements for, the above 10^(th) and11^(th) embodiments are now explained.

A light beam incident on the first lens group is flipped up by thedivergence of the first lens. In the eyepiece optical system and therelay type finder optical system according to the invention, theoff-axis light beam is refracted through the three positive lensessubsequent to the first lens. Here, as the space between the first lensand the second lens becomes wide, the off-axis light beam incident onthe second, third and fourth lenses gains height, incurring an increasein the size of the eyepiece optical system and having considerableinfluences on off-axis aberrations and aberrations due to decentration.

Therefore, if the first lens and the second lens are cemented togetherinto a cemented doublet, then the space between the first lens and thesecond lens can be minimized, contributing to decreasing the size of theeyepiece optical system and reducing its sensitivity to decentration.

The 12^(th) embodiment according to the fourth aspect of the inventionis characterized in that the relay type finder optical system accordingto the 8^(th) embodiment satisfies the following conditions:

2.5≦f123/fA≦6.5  (4-1)′

1.45≦ff/fA≦2  (4-2)′

where:

f123 is the composite focal length of the first lens, the second lensand the third lens,

f4 is the focal length of the fourth lens, and

fA is the focal length of the eyepiece optical system.

The 13^(th) embodiment according to the fourth aspect of the inventionis characterized in that the eyepiece optical system according to the9^(th) embodiment satisfies the following conditions:

2.5≦f123/fA≦6.5  (4-1)′

1.4≦f4/fA≦2  (4-2)′

where:

f123 is the composite focal length of the first lens, the second lensand the third lens,

f4 is the focal length of the fourth lens, and

fA is the focal length of the eyepiece optical system.

The advantages of, and the requirements for, the above 12^(th) and13^(th) embodiments are now explained.

With these embodiments, it is possible to easily obtain a balancebetween reducing the changes of magnification during the diopter controland obtaining a long eye point over a wide angle-of-field range. Inaddition, refracting power balances and aberration balances areimproved, too.

In accordance with the fourth aspect of the invention, it is possible toattain an eyepiece optical system that has a relatively wide field, along eye relief from an eyepiece lens to an eye point and a compacttotal length including the position of the image being viewed, which isformed by an image-formation lens.

It is also possible to have an eyepiece optical system that iscompatible with a single-lens reflex camera using a relatively smallimage pickup device and suited for use with a compact relay type finderoptical system, and a finder optical system for single-lens reflexcameras, which incorporates it.

Next, reference is made to the fifth aspect of the invention.

The first embodiment according to the fifth aspect of the invention isdirected to a relay type finder optical system comprising a relayoptical system operable to re-form a subject image at a secondaryimage-formation position wherein the subject image is a primary imageformed at a primary image-formation position through an objectiveoptical system and an eyepiece optical system operable to view an imagere-formed via the relay optical system, characterized in that:

the relay type finder optical system comprises at least three reflectingsurfaces between the primary image-formation position and the secondaryimage-formation position, and

of the at least three reflecting surfaces, two reflecting surfaces lyingside by side on an optical axis in an optical path through the relayoptical system are internal reflecting surfaces of a one-piece prism P1,wherein two such reflecting surfaces are total-reflection surfaces.

The advantage of, and the requirement for, the above first embodimentarrangement is now explained.

In a relay type finder optical system, given paraxial distances must bebetween the primary image-formation position and the principal points ofa relay optical system and between the principal points of the relayoptical system and the secondary image-formation position. When acompact layout is figured out, one possible option is to bend an opticalpath between the primary image-formation position and the secondaryimage-formation position.

In the invention, at least three reflecting surfaces are located betweenthe primary image-formation position and the secondary image-formationposition, whereby the layout for the relay optical system can be keptcompact.

In this case, however, care must be taken of a decrease in the quantityof light at the reflecting surfaces and precise location of the anglesbetween the reflecting surfaces.

In the invention, two out of the reflecting surfaces located between theprimary image-formation position and the secondary image-formationposition are defined by internal surfaces of a one-piece prism, therebygetting around angle errors at the time of assembling. Here, if two suchinternal reflecting surfaces are defined by total-reflection surfaces,the decrease in the quantity of light can then be held back.

Preferably, five reflecting surfaces should be located on the viewerside with respect to the primary image-formation position, because areasonable tradeoff is easily achievable between compactness and gettinghold of light quantity.

The second embodiment according to the fifth aspect of the invention isdirected to a relay type finder optical system comprising a relayoptical system operable to re-form a subject image at a secondaryimage-formation position wherein the subject image is a primary imageformed at a primary image-formation position through an objectiveoptical system and an eyepiece optical system operable to view an imagere-formed via the relay optical system, characterized in that:

the relay type finder optical system comprises at least three reflectingsurfaces between the primary image-formation position and the secondaryimage-formation position, and

of the at least three reflecting surfaces, two reflecting surfaces lyingside by side on an optical axis in an optical path through the relayoptical system are internal reflecting surfaces of a one-piece prism P1,wherein two such reflecting surfaces are total-reflection surfaces, andthe angle of incidence of the optical axis on two such internalreflecting surfaces is greater than 45°.

The advantage of, and the requirement for, the above second embodimentarrangement is now explained.

In a relay type finder optical system, given paraxial distances must bebetween the primary image-formation position and the principal points ofa relay optical system and between the principal points of the relayoptical system and the secondary image-formation position. When acompact layout is figured out, one possible option is to bend an opticalpath between the primary image-formation position and the secondaryimage-formation position.

In the invention, at least three reflecting surfaces are located betweenthe primary image-formation position and the secondary image-formationposition, whereby the layout for the relay optical system can be keptcompact.

In this case, however, care must be taken of a decrease in the quantityof light at the reflecting surfaces and precise location of the anglesbetween the reflecting surfaces.

In the invention, two of the reflecting surfaces located between theprimary image-formation position and the secondary image-formationposition are defined by internal surfaces of a one-piece prism, therebygetting around angle errors at the time of assembling. Here, if theangle of incidence of the optical axis on two such internal reflectingsurfaces is set at greater than 45°, the decrease in the quantity oflight can then be held back.

Preferably, five reflecting surfaces should be located on the viewerside with respect to the primary image-formation position, because areasonable tradeoff is easily achievable between compactness and gettinghold of light quantity.

The third embodiment according to the fifth aspect of the invention ischaracterized in that in the 1^(st) or the 2^(nd) embodiment,

the number of reflecting surfaces between the objective optical systemand the primary image-formation position is an even number,

the total of reflecting surfaces located between the subject imageformed at the primary image-formation position and the image re-formedat the secondary image-formation position is five,

a positive lens group RL comprising at least one positive lens islocated between the primary image-formation position and the secondaryimage-formation position,

two or more out of the five reflecting surfaces are located on the sideof the positive lens group RL facing a primary image-formation plane,

two or more out of the five reflecting surfaces are located on the sideof the positive lens group RL facing a secondary image-formation plane,

the prism P1 comprises a set PM of back-to-back two reflecting surfacesin the five reflecting surfaces, and

one out of the five reflecting surfaces is located between the positivelens group RL and the set PM of two reflecting surfaces.

The advantage of, and the requirement for, the above third embodimentarrangement is now explained.

With the relay type finder optical system including an objective opticalsystem, an even number of reflections could be implemented to obtain anerected image.

If five reflecting surfaces are located between the primaryimage-formation position and the secondary image-formation position and,at the same time, two or more reflecting surfaces are located on eachside of the positive lens group RL that forms the whole or a part of therelay optical system, it is then possible to attain a decreasedtotal-length, compact layout including the eyepiece optical system.

Here, if one reflecting surface is located between the set PM of tworeflecting surfaces in the prism P1 and the positive lens group RL, itis then possible to attain a more compact layout.

The prism P1 could be such that it has only one set PM of two internalreflecting surfaces or, alternatively, it could have one set PM of twointernal reflecting surfaces plus three of the aforesaid reflectingsurfaces.

Preferably for the purpose of improving on the quality of the imagebeing viewed, the positive lens group RL should have at least onepositive lens, at least one negative lens and at least one positive lensas viewed from the primary image-formation plane side.

The fourth embodiment according to the fifth aspect of the invention ischaracterized in that in any one of the 1^(st) to the 3^(rd) embodiment,

the total of reflecting surfaces located between the subject imageformed at the primary image-formation position and the image re-formedat the secondary image-formation position is five,

a positive lens group RL comprising at least one positive lens islocated between the primary image-formation position and the secondaryimage-formation position,

two or more out of the five reflecting surfaces are located on the sideof the positive lens group RL facing a primary image-formation plane,

two or more out of the five reflecting surfaces are located on the sideof the positive lens group RL facing a secondary image-formation plane,

the prism P1 comprises a set PM of back-to-back two reflecting surfacesin the five reflecting surfaces,

one out of the five reflecting surfaces is located between the positivelens group RL and the set PM of two reflecting surfaces, and

a reflecting surface of the two reflecting surfaces with the positivelens group RL intervened, wherein said reflecting surface faces away aprism P1 side, is operable to reflect an optical axis at an acute angleand at least one positive lens SR is located between the two or morereflecting surfaces faces away a positive lens group RL side on whichthe prism P1 is located.

The advantage of, and the requirement for, the above fourth embodimentarrangement is now explained.

If five reflecting surfaces are located between the primaryimage-formation position and the secondary image-formation position and,at the same time, two or more reflecting surfaces are located on eachside of the positive lens group RL that forms the whole or a part of therelay optical system, it is then possible to attain a decreasedtotal-length, compact layout including the eyepiece optical system.

Here, if one reflecting surface is located between the set PM of tworeflecting surfaces in the prism P1 and the positive lens group RL, itis then possible to attain a more compact layout.

Further, if at least one of the two reflecting surfaces with thepositive lens group RL intervened is operable to reflect an optical axisat an acute angle, it is then possible to attain a more compact layout.In this arrangement, that surface cannot be configured into atotal-reflection surface. However, because the positive lens SR can bepositioned between the two or more reflecting surfaces including anacute reflecting surface, the image-formation capability and totallength of the relay optical system can be well adjusted. If a part ofthe condenser function is given to that positive lens SR, the wholefinder can be kept compact with high performance.

The prism P1 could be such that it has only one set PM of two internalreflecting surfaces or, alternatively, it could have one set PM of twointernal reflecting surfaces plus three of the aforesaid reflectingsurfaces.

Preferably for the purpose of improving on the quality of the imagebeing viewed, the positive lens group RL should have at least onepositive lens, at least one negative lens and at least one positive lensas viewed from the primary image-formation plane side.

The fifth embodiment according to the fifth aspect of the invention ischaracterized in that in any one of the 1^(st) to the 4^(th) embodiment,the prism P1 is located on the side of the positive lens group RL facingthe primary image-formation plane.

The advantage of, and the requirement for, the above 5^(th) embodimentarrangement is now explained.

With that arrangement, the relay type finder optical system is easilyconfigured such that its size does not grow large toward the subjectside (that faces away the viewer side). It is thus easy to set up agenerally compact single-lens reflex camera or a single-lens reflexcamera that provides no or little restriction on the objective lens tobe used.

The sixth embodiment according to the fifth aspect of the invention ischaracterized in that the optical function surfaces of the prism P1 areeach composed of a plane.

The advantage of, and the requirement for, the above 6^(th) embodimentarrangement is now explained. With that arrangement, it is possible toimprove on the productivity of the prism P1 and its ability to be builtin a camera body.

The seventh embodiment according to the fifth aspect of the invention ischaracterized in that the prism P1 has optical power.

The advantage of, and the requirement for, the above 7^(th) embodimentarrangement is now explained.

If optical power is given to the prism P1, it is then easy to set up afinder system that is of higher performance, composed of fewer lenses orof more compact size. If power is given to the prism P1 at a positionnear to the positive lens group RL, it then contributes to theperformance of the relay optical system, and if power is given to theprism P1 at a position near to the primary or secondary image-formationplane, it then contributes to a condenser function. The power could begiven to the entrance or exit surface in the form of a lens surface orthe reflecting surface. Note here that when the power is imparted to thereflecting surface, it is preferable to apply a rotationally asymmetriclens function surface to at least another surface.

The eighth embodiment according to the fifth aspect of the invention ischaracterized in that in any one of the 1^(st) to the 5^(th) embodiment,an optical axis exiting from the prism P1 is more inclined toward thesubject side than an optical axis incident on the prism P1.

The advantage of the above 8^(th) embodiment arrangement is that itworks for compactness.

The ninth embodiment according to the fifth aspect of the invention ischaracterized in that in any one of the 1^(st) to the 5^(th) embodiment,

on a side of the primary image-formation position on which light raysare incident, there is located one optical path splitter reflectingsurface operable to reflect an optical axis,

the reflecting surfaces located between the primary image-formationposition and the second image-formation position are defined by fivereflecting surfaces including, in order of a reflection optical path, afirst reflecting surface, a second reflecting surface, a thirdreflecting surface, a fourth reflecting surface and a fifth reflectingsurface, and

when the optical axis of the relay type finder optical system isprojected on a plane including optical axes incident on and reflected atthe optical path splitter reflecting surface with the direction ofreflection of light at the optical path splitter reflecting surface setas a counterclockwise direction, the first reflecting surface reflectsan optical axis in the counterclockwise direction, an optical axis isbent at the second reflecting surface, the third reflecting surface andthe fourth reflecting surface in a clockwise direction, and an opticalaxis is bent at the fifth reflecting surface in the counterclockwisedirection.

The advantage of, and the requirement for, the above 9^(th) embodimentarrangement is now explained. With that arrangement, the optical pathfrom the primary image-formation position up to the secondaryimage-formation position can be compactly folded back.

The 10^(th) embodiment according to the fifth aspect of the invention isdirected to a single-lens reflex camera characterized by comprising therelay type finder optical system according to any one of the 1^(st) tothe 9^(th) embodiment.

The advantage of, and the requirement for, the above 10^(th) embodimentarrangement is now explained. By relying upon the relay type finderoptical system of the invention, the optical path through the finderoptical system can be turned back with fewer reflecting surfaces. Thus,a single-lens reflex camera that is compact yet reduced in terms oflight quantity drops can be set up albeit having a secondaryimage-formation type finder. Further, a single-lens reflex camera havinga high finder magnification can be built up.

In accordance with the fifth aspect of the invention, it is possible toachieve a relay type finder optical system wherein reflecting surfacesare located before and after a lens element in a relay optical system sothat it can be kept compact in both the height direction of a finderoptical system and the optical axis direction of an objective opticalsystem while decreases in the quantity of light are held back. It isalso possible to provide a relay type finder optical system that iscompatible even with a single-lens reflex camera using a relativelysmall image pickup device, and compact with a little light quantity dropas well, and a single-lens reflex camera incorporating it.

Specific embodiments are given of the inventive optical system, finderoptical system, relay type finder optical system, eyepiece opticalsystem and single-lens reflex camera in this order.

The first aspect of the invention is now explained with reference to theembodiments of the optical system.

First of all, examples of how to shift the whole relay lens RL foranti-shake purposes are explained.

FIG. 1 is an optical path diagram for the finder optical system commonto Embodiments 1-5, from which a taking optical system and an opticalpath splitter means for splitting an optical path into the finderoptical system are removed (in this regard, see FIG. 3).

In this finder optical system, a primary image formed through the takingoptical system (not shown) is formed on a primary image-formation planeI₁, then re-formed as a secondary image on a secondary image-formationplane I₂ via a relay optical system RLS, and viewed as a virtual imagethrough an eyepiece lens EP. In the optical path diagram of FIG. 1, thatvirtual image is formed as a real image on an image plane I₃ through anideal lens IDL.

In that finder optical system, the relay optical system RLS is made upor six groups or seven lenses, specifically, a positive meniscus lens L₁convex on its object side, a positive meniscus lens L₂ convex on itsobject side, a double-concave negative lens L₃, a cemented doubletconsisting of a negative meniscus lens L₄ convex on its object side anda double-convex positive lens L₅, a double-convex positive lens L₆ and aplano-convex positive lens L₇, wherein a relay lens RL is built up offive groups or six lenses, i.e., lenses L₁ to L₆, and the plano-convexpositive lens L₇ is a field lens. The secondary image-formation plane I₂is in alignment with the image-side plane position of the plano-convexpositive lens L₇. Note here that the object-side surface of the positivemeniscus lens L₁, both surfaces of the double-convex positive lens L₆and the object-side surface of the plano-convex positive lens L₇ areeach defined by an aspheric surface. The eyepiece lens EP is made up ofthree groups or four lenses, specifically, a cemented doublet consistingof a double-concave negative lens L₈ and a double-convex positive lensL₉, a double-convex positive lens L₁₀ and a double-convex positive lensL₁₁. In FIG. 1, note that a plane-parallel plate group located betweenthe primary image-formation plane I₁ and the relay optical system RLS,for instance, is an optical path bending prism, and a stop S is locatedat an eye point position between the eyepiece lens EP and the ideal lensIDL.

Lens data on this finder optical system will be given later.

FIG. 2 is illustrative of one exemplary actual layout. In this finderoptical system, the optical path is bent by an optical path bendingprism PR and mirrors M₁, M₂ and M₃ to keep it compact. These areindicated in a straight line optical path diagram form in FIG. 1.

In addition to the modification of FIG. 2, there are some possiblemodifications to FIG. 1. For instance or preferably, if the field lensFL is spaced far away from the secondary image-formation plane I₂, dustdeposited onto a focusing screen, a field frame or the like is much lessvisible. Alternatively, as shown in FIG. 3, the entrance or exit surfaceof the optical path bending prism PR could be configured as a lens LF.In FIG. 3, the entrance surface of the optical path bending prism PR isconfigured as a convex surface that has a field lens function for theprimary image-formation plane I₁. Note here that PLS stands for a takingoptical system. This taking optical system PLS comprises an optical pathsplitter means DM for splitting the optical path into the finder opticalsystem, and is operable to form a primary image of the subject on theprimary image-formation plane I₁ via the optical path splitter means DM.

In the finder optical system of FIG. 1, some lenses L₈, L₉ and L₁₀ ofthe lenses forming the eyepiece lens EP are moved in the optical axisdirection to implement diopter control. With a position of −1 diopterset as a reference, a 2.18 mm movement of L₈, L₉ and L₁₀ toward thesecondary image-formation plane I₂ results in −3 diopter, and a 2.16 mmmovement of L₈, L₉ and L₁₀ in such a way to face away from the secondaryimage-formation plane I₂ results in +1 diopter.

FIG. 9 is a collection of transverse aberration diagrams for ashake-free state on a real image plane I₃ formed by the ideal lens IDL.In this aberration diagrams, angles at the center indicate angles ofview in the vertical direction; FIG. 9 is indicative of transverseaberrations at those angles in the meridional and sagittal directions.The same shall apply hereinafter.

Next, consider the case where the finder optical system of FIG. 1 isinclined with the position of the primary image-formation position I₁shifted 0.5 mm down. Then, the position of the secondary image-formationplane I₂ is shifted 1.43 mm up. Here, as the relay lens RL is shifted0.23 mm down in FIG. 1, it allows an image misalignment on the secondaryimage-formation plane I₂ to be so corrected that a shake-free image canbe viewed.

FIG. 10 is illustrative of transverse aberrations after anti-shakeoperation in Embodiment 1. From a comparison with FIG. 9, it is seenthat underperformance due to anti-shake operation is reduced.

As in this embodiment 1, it is preferable to isolate the diopter controllens (some lenses L₈, L₉ and L₁₀ of the lenses forming the eyepiece lensEP) from the anti-shake lens (RL), because it is easy to get hold ofprecision of both and locate actuators for both.

Embodiment 2, i.e., an example of how to shift lenses L₁ to L₅ formingthe relay lens RL for anti-shake purposes is now explained.

Consider here the case where the optical system of FIG. 1 is inclinedwith the position of the primary image-formation plane I₁ shifted 0.5 mmdown. Then, the position of the secondary image-formation plane I₂ isshifted 0.43 mm up. Here, as lenses L₁ to L₅ forming the relay lens RLare shifted 0.32 mm down in FIG. 1, it allows an image misalignment onthe secondary image-formation plane I₂ to be so corrected that ashake-free image can be viewed.

FIG. 11 is illustrative of transverse aberrations after anti-shakeoperation in Embodiment 2. From a comparison with FIG. 9, it is seenthat underperformance due to anti-shake operation is reduced.

In this embodiment, the anti-shake operation is implemented with thelenses L₁ to L₅ other than the lens L₇ that is the field lens and thelens L₆ that has a role analogous to the field lens; the effectivediameter and weight of the anti-shake group can be reduced.

It is preferable to shift the optical system including the pupil (thatis formed near the lens L₃) for anti-shake purposes as in thisembodiment, because the effective diameter of the optical system nearthe pupil and the weight of the anti-shake group can be reduced tolessen loads on an actuator for shifting the anti-shake group.

Note here that this embodiment could be modified such that another fieldlens is added on the primary image-formation plane I₁ side with respectto the anti-shake group so as to obtain a further reduction in theeffective diameter of the anti-shake group. The same shall apply to thefollowing embodiments.

Embodiment 3, i.e., an example of how to shift lenses L₄ to L₆ formingthe relay lens RL for anti-shake purposes as shown in FIG. 6 is nowexplained.

Consider here the case where the optical system of FIG. 1 is inclinedwith the position of the primary image-formation plane I₁ shifted 0.5 mmdown. Then, the position of the secondary image-formation plane I₂ isshifted 0.43 mm up. Here, as lenses L₄ to L₆ forming the relay lens RLare shifted 0.21 mm down in FIG. 1, it allows an image misalignment onthe secondary image-formation plane I₂ to be so corrected that ashake-free image can be viewed.

FIG. 12 is illustrative of transverse aberrations after anti-shakeoperation in Embodiment 3. From a comparison with FIG. 9, it is seenthat underperformance due to anti-shake operation is reduced.

A comparison of Embodiment 3 with 2 teaches that the transversemagnification β_(a) of the anti-shake group plunges from 1.82 down to1.04, but the transverse magnification β_(b) of the relay lens group onthe image side with respect to the anti-shake group climbs from 0.47 upto 0.98. As a result, the anti-shake sensitivity jumps from 1.33 up to2.0.

Embodiment 4, i.e., an example of how to shift lenses L₄ to L₅ formingthe relay lens RL for anti-shake purposes as shown in FIG. 7 is nowexplained.

Consider here the case where the optical system of FIG. 1 is inclinedwith the position of the primary image-formation plane I₁ shifted 0.5 mmdown. Then, the position of the secondary image-formation plane I₂ isshifted 0.43 mm up. Here, as a cemented doublet composed of lenses L₄ toL₅ forming the relay lens RL are shifted 0.29 mm down in FIG. 1, itallows an image misalignment on the secondary image-formation plane I₂to be so corrected that a shake-free image can be viewed.

FIG. 13 is illustrative of transverse aberrations after anti-shakeoperation in Example 4. From a comparison with FIG. 9, it is seen thatunderperformance due to anti-shake operation is reduced.

As in this embodiment, it is preferable for the anti-shake group toinclude a positive lens and a negative lens for correction of chromaticaberrations, because deterioration in chromatic aberrations is reducedduring the anti-shake operation. Here, if the anti-shake group includesa cemented doublet composed of a positive lens and a negative lens, itis then more preferable because of being less likely to be affected byfabrication errors such as thickness errors and decentration errors.

Embodiment 5, i.e., an example of how to shift the eyepiece lens EP foranti-shake purposes as shown in FIG. 8 is now explained.

Consider here the case where the optical system of FIG. 1 is inclinedwith the position of the primary image-formation plane I₁ shifted 0.5 mmdown. Then, the position of the secondary image-formation plane I₂ isshifted 0.43 mm up. Here, as the eyepiece lens EP is shifted 0.44 mmdown in the drawing, it allows an image misalignment on the secondaryimage-formation plane I₂ to be so corrected that a shake-free image canbe viewed.

FIG. 14 is illustrative of transverse aberrations after anti-shakeoperation in Embodiment 5. From a comparison with FIG. 9, it is seenthat underperformance due to anti-shake operation is reduced.

Lens data on the finder optical system of FIG. 1 is given as NumericalEmbodiment 0, wherein r₁, r₂, etc. are the radii of curvature of therespective lens surfaces, d₁, d₂, etc. are the spaces between adjacentlens surfaces, n_(d1), n_(d2), etc. are the d-line refractive indices ofthe respective lens surfaces, and v_(d1), v_(d2), etc. are the Abbeconstants of the respective lenses. Here let x represent an optical axiswith the proviso that the direction of travel of light is taken aspositive, and y stand for a direction orthogonal to the optical axis.Then, aspheric shape is given by

x=(y ² /r)/[1+{1−(K+1)(y/r)²}^(1/2) ]+A ₄ y ⁴

where r is a paraxial radius of curvature, K is a conical coefficient,and A₄ is the fourth-order aspheric coefficient.

Numerical embodiment: 0 r₁ = ∞ (I₁) d₁ = 7.00 n_(d1) = 1.5163 ν_(d1) =64.1 r₂ = ∞ d₂ = 10.00 n_(d2) = 1.5163 ν_(d2) = 64.1 r₃ = ∞ d₃ = 22.80n_(d3) = 1.5163 ν_(d3) = 64.1 r₄ = ∞ d₄ = 10.80 n_(d4) = 1.6935 ν_(d4) =53.2 r₅ = ∞ d₅ = 16.80 n_(d5) = 1.8040 ν_(d5) = 46.6 r₆ =14.94(Aspheric) d₆ = 4.72 n_(d6) = 1.8467 ν_(d6) = 23.8 r₇ = 200.43 d₇ =0.44 n_(d7) = 1.7174 ν_(d7) = 29.5 r₈ = 14.06 d₈ = 4.81 n_(d8) = 1.8040ν_(d8) = 46.6 r₉ = 25.38 d₉ = 2.84 n_(d9) = 1.5254 ν_(d9) = 55.8 r₁₀ =−14.50 d₁₀ = 1.10 n_(d10) = 1.5254 ν_(d10) = 55.8 r₁₁ = 10.42 d₁₁ = 8.92n_(d11) = 1.8467 ν_(d11) = 23.8 r₁₂ = 300.05 d₁₂ = 1.60 n_(d12) = 1.6031ν_(d12) = 60.6 r₁₃ = 26.50 d₁₃ = 5.46 n_(d13) = 1.7859 ν_(d13) = 44.2r₁₄ = −22.88 d₁₄ = 22.93 n_(d14) = 1.8340 ν_(d14) = 37.2 r₁₅ =30.04(Aspheric) d₁₅ = 5.55 r₁₆ = −276.45(Aspheric) d₁₆ = 21.67 r₁₇ =56.07(Aspheric) d₁₇ = 3.05 r₁₈ = ∞ d₁₈ = 0.00 r₁₉ = ∞ (I₂) d₁₉ = 12.48r₂₀ = −14.70 d₂₀ = 1.37 r₂₁ = 66.77 d₂₁ = 6.56 r₂₂ = −20.00 d₂₂ = 0.50r₂₃ = 244.69 d₂₃ = 3.62 r₂₄ = −39.79 d₂₄ = 3.36 r₂₅ = 37.54 d₂₅ = 3.30r₂₆ = ∞ d₂₆ = 23.00 r₂₇ = ∞ (S) d₂₇ = 31.62 r₂₈ = ∞ (IDL) d₂₈ = 32.62r₂₉ = ∞ (I₃) Aspherical Coefficients 6th surface K = 1.1550 × 10⁻¹ A₄ =6.4822 × 10⁻⁶ 15th surface K = −8.4040 × 10⁻¹ A₄ = 1.3875 × 10⁻⁵ 16thsurface K = 8.4052 × 10⁻¹ A₄ = 1.2911 × 10⁻⁵ 17th surface K = −6.6270 ×10⁻¹ A₄ = 3.0506 × 10⁻⁵

Tabulated below are the values of conditions (1-1), (1-2) and (1-3) inEmbodiments 1, 2, 3 and 4.

Condition (1-1) (1-2) (1-3) Embodiment 1 1.857 1.000 −0.857 Embodiment 21.328 2.948 −1.820 Embodiment 3 2.004 5.002 −1.040 Embodiment 4 2.4934.802 −2.170 Embodiment 5 −0.974 — —

The finder optical system and single-lens reflex camera according to thesecond aspect of the invention are now explained with reference to someembodiments.

FIG. 15 is illustrative in section of the construction of a single-lensreflex camera that incorporates the finder optical system according toone example of the invention. In FIG. 15, reference numeral 1 stands fora single-lens reflex camera that incorporates the finder optical systemaccording to one embodiment of the invention, and reference numeral 2indicates an inter-changeable taking lens. Note here that the takinglens 2 could be made integral with a camera body.

Reference is now made with reference to the order of travel of a lightbeam emanating from a subject and exiting from the taking lens 2.

A light beam emanating from the taking lens 2 is reflected at a quickreturn mirror 11 at an angle of reflection of 90° in this embodiment.Note here that when a half-silvered mirror is used as the quick returnmirror 11, the light beam transmitting through it could be guided to afocal detection means (not shown).

In what follows, on the basis of the position where the optical axisexiting from the taking lens 2 is reflected at the quick return mirror11, a direction which is vertical to the optical axis exiting from thetaking lens 2 and in which the optical axis is reflected at the quickreturn mirror 11 will be referred to as an upward direction, a directionwhich is parallel with the optical axis of the taking lens 2 and inwhich the taking lens 2 is located will be called a subject direction,and a direction reverse to the direction of locating the taking lens 2will be called a viewer direction.

The light beam reflected at the quick return mirror 11 is incident on afocal plane plate 12 placed at a position optically equivalent(conjugate) to an image pickup device 103 to be described later.Referring here to FIG. 16 to be described later, when a subject image isformed on the image pickup device 103, it is also formed on the focalplane plate 12. The focal plane plate 12 could also have a condenserlens function.

The light beam exiting from the focal plane plate 12 is incident on aprism 13. The prism 13 has an entrance surface 13 a, a reflectingsurface 13 b, a reflecting surface 13 c and an exit surface 13 d.Desirously, the entrance surface 13 a is vertical to the axis ofincident light. The light beam incident on the entrance surface 13 a isreflected at the reflecting surface 13 b in the subject direction. Lightquantity losses here can be reduced by the satisfaction of thetotal-reflection condition. The reflecting surface 13 b is tantamount tothe aforesaid F3 reflecting surface. Further, the light beam isreflected in the upward direction such that the optical axis hascomponents in the upward direction and subject direction at thereflecting surface 13 c (that is, the optical axis direction liesbetween the subject direction and the upward direction). Light quantitylosses here can be reduced by the satisfaction of the total-reflectioncondition. The reflecting surface 13 c is tantamount to the aforesaid F2reflecting surface. Further, the light beam leaves the prism 13 throughthe exit surface 13 d. The exit surface 13 d here is desirously verticalto the optical axis.

The light beam exiting from the prism 13 is further reflected at amirror 14 in the viewer direction, with the optical axis substantiallyparallel with the optical axis of the taking lens 2. The mirror 14 istantamount to the aforesaid F1 reflecting surface.

Then, the light beam transmits through lens 21, lens 22, lens 23 andlens 24 where it is subjected to lens actions. In Embodiment 1 of FIG.15, the lens 21 is a positive meniscus lens tightly convex on itssubject side, the lens 22 is a positive meniscus lens tightly convex onits subject side, the lens 23 is a double-concave negative lens, and thelens 24 is a cemented doublet composed of a negative meniscus lensconvex on its subject side and a double-convex positive lens and havinga positive composite power.

The lens groups here are arranged such that their composite power turnspositive, and must work as a substantial part of the relay opticalsystem. Specific lens arrangement should preferably be designed whiletaking aberrations, etc. into account.

Preferably, at least one positive lens (corresponding to the lens 21 or22 in Embodiment 1 of FIG. 15) is provided, at least one negative lens(corresponding to the lens 23 in Embodiment 1 of FIG. 15) is located onthe side of that positive lens facing the R1 reflecting surface, and atleast one positive lens (corresponding to the positive lens providingthe lens 24 in Embodiment 1 of FIG. 15) is located on the side of thatnegative lens facing the R1 reflecting surface, whereby the principalpoints can be set within this region and given magnification and opticalperformance can be easily attainable as well. Note here that an aperturestop 31 is located near that negative lens for efficient pupiltransmission.

Next, the light beam is reflected at a mirror 15 to bend the opticalaxis in the subject direction and a downward direction. The mirror 15 istantamount to the aforesaid R1 reflecting surface.

Further, the optical axis is reflected at a mirror 16 in the viewerdirection. Here, a positive lens 25 is located between the mirrors 15and 16, thereby improving on the image-formation capability and pupiltransmission capability of the relay optical system.

The light beam reflected at the mirror 16 is incident on a condenserlens 26 placed near a secondary image-formation position 17. InEmbodiment 1 of FIG. 15, the condenser lens 26 is a plano-convexpositive lens convex on its subject side, with its plane sidesubstantially in alignment with the secondary image-formation position17.

Further, the light beam passes through a dust-preventive glass 32. Thedust-preventive glass 32 cooperates with other frame or the like toprevent dusts and other contaminants from deposition onto lens surfacesnear the secondary image-formation position 17.

Further, the light beam is subjected to lens actions at lens 27, lens 28and lens 29 forming the eyepiece optical system, exiting from a camerabody. Finally, the light beam is guided to the eye of the viewer.

FIG. 16 is illustrative of in what state the single-lens reflex cameraof FIG. 15 is operated. Note however that the outline indicative of acamera body is not drawn.

Upon operation, the quick return mirror 11 is retracted back from theoptical path, and a light beam leaving the taking lens 2 transmitssuccessively through a filter 101 and a filter 102, entering the imagepickup device 103. The filers 101 and 102 are each imparted with somefunctions such as an infrared cut filter function, a low-pass filterfunction and a dust-preventive filter function, and their number is notnecessarily limited to two. The image pickup device 103 is an electronicimage pickup device such as CCS or C-MOS, or a silver halide film.

FIG. 17 is a more schematic representation of FIG. 15, indicative of theaforesaid angles α_(f), θ_(f), θ_(m), θ_(r) and α_(r).

FIG. 18 is illustrative in schematic of a specific embodiment whereinthe quick return mirror of FIGS. 15-17 is replaced by a prism group 41having a half-silvered mirror surface 41 a. This embodiment is otherwisethe same as those of FIGS. 15-17, and so will not be detailed (about thelens system in particular). With this embodiment, the finder can be usedto check up a subject and, at the same time, take an image of thesubject.

FIG. 19 is illustrative of a modification to the single-lens reflexcamera 1 shown in FIG. 15, wherein two plane mirrors 13 b′ and 13 c′ areused in place of the prism 13.

Of the two plane mirrors 13 b′ and 13 c′, the reflecting mirror 13 b′ onthe primary image-formation plane (focal plane plate 12) sidecorresponds to the F3 reflecting surface, and the other reflectingsurface 13 c′ corresponds to the F2 reflecting surface. The angle ofreflection of light at the two plane mirrors 13 b′ and 13 c′ lies in thesame direction as is the case with the reflecting surfaces 13 b and 13 ain FIG. 15. That modification is otherwise the same as in FIGS. 15-17,and so will not be detailed.

FIG. 20 is illustrative of a modification to the single-lens reflexcamera of FIG. 15, wherein the prism 13 is dispensed with, and the angleof a quick return mirror 61 with respect to the optical axis duringviewing is acute.

A light beam reflected at the quick return mirror 61 is incident on afocal plane plate 62 placed at the primary image-formation position. Thelight beam passing through the focal plane plate 62 is incident alongthe optical axis on a mirror 63 (corresponding to the F1 reflectingsurface).

The optical axis incident on the mirror 63 is reflected there at anacute angle in the viewer direction and the upward direction such thatit comes parallel with and close to the optical axis of the taking lens2. Then, the light beam passes through the lens 21, lens 22, lens 23 andlens 24 shown in FIG. 15, where it is subjected to lens actions.

The axis of incident light on a mirror 64 (the R1 reflecting surface) isreflected there at an acute angle in the subject direction and thedownward direction.

The surfaces of the mirrors 63 and 64 subtend at an acute angle.

The optical axis is reflected at the mirror 65 in the viewer direction.Here, if the positive lens 25 is located between the mirrors 64 and 65,it is then possible to improve on the image-formation capability andpupil transmission capability of the relay optical system.

The light beam reflected at the mirror 65 is incident on the condenserlens 26 located near the secondary image-formation position 17. In FIG.20, the condenser lens 26 is a plano-convex positive lens convex on thesubject side with a plane side substantially in alignment with thesecondary image-formation position 17.

Further, the light beam passes through the dust preventive glass 32. Thedust preventive glass 32 cooperates with other frame, etc. to preventdust or the like from deposition onto a lens surface near the secondaryimage-formation position 17.

Finally, the light beam is subjected to lens actions at the lens 27, thelens 28 and the lens 29 that form together an eyepiece optical system,leaving the camera body and arriving at the eye of the viewer.

With such arrangement, the optical axis incident on the mirror 63(tantamount to the aforesaid F1 reflecting surface) can be inclined inthe subject direction without recourse to the prism 13 of FIG. 15.

FIG. 21 is a more schematic representation of FIG. 20, indicating thatit is only required to locate a lens group 71 of positive power betweenthe mirrors 63 and 64 and a lens group 72 of positive power between themirror 65 and the viewer. Note that the secondary image-formationposition is preferably found between the mirror 65 and the lens group72.

Numerical Embodiments 1 and 2 of the lens system contemplated in FIG. 15are now given. Note that the lens surface shapes in FIG. 15 correspondto those in Numerical Embodiment 1.

FIG. 22 is a sectional view of the finder optical system of NumericalEmbodiment 1 as taken apart along its optical axis, with its numericaldata given later.

In FIG. 22, the optical members in FIG. 15, too, are indicated byreference numerals. Referring specifically to this, the surface r₁corresponds to the primary image-formation position on the focal planeplate 12, and the surfaces r₂ to r₅ correspond to the prism 13: r₂, r₃,r₄ and r₅ correspond to the entrance surface 13 a, the reflectingsurface 13 b, the reflecting surface 13 c and the exit surface 13 d,respectively.

The surface r₆ corresponds to the mirror 14, the surface r₇ to r₈ to thelens 21, the surface r₉ to r₁₀ to the lens surface 22, the surface r₁₁to the aperture stop 31, the surface r₁₂ to r₁₃ to the lens surface 23,and the surface r₁₄ to r₁₆ to the lens 24.

Likewise, the surface r₁₇ corresponds to the mirror 15, the surfaces r₁₈to r₁₉ to the positive lens 25, the surface r₂₀ to the mirror 16, thesurfaces r₂₁ to r₂₂ to the condenser lens 26, and the surface r₂₃ inalignment with the surface r₂₂ to the secondary image-formation position17.

Finally, the surfaces r₂₄ to r₂₅ corresponds to the dust preventiveglass 32, the surfaces r₂₆ to r₂₈ to the lens 27 that provides aneyepiece optical system, the surfaces r₂₉ to r₃₀ to the lens 28, thesurfaces r₃₁ to r₃₂ to the lens 29, and the surface r₃₃ to the pupil ofthe viewer, viz., the eye point EP.

In this embodiment, the relay optical system is made up of lenses 21,22, 23, 24, 25 and 26, wherein the lens 21 is a positive meniscus lensconvex on the primary image-formation position side, the lens 22 is apositive meniscus lens convex on the primary image-formation positionside, the lens 23 is a double-concave negative lens, the lens 24 is acemented lens consisting of a negative meniscus lens convex on theprimary image-formation position side and a double-convex positive lens,the lens 25 is a positive meniscus lens convex on the primaryimage-formation position side, and the lens 26 is a condenser lensconsisting of a plano-convex positive lens. The eyepiece optical systemis made up of lenses 27, 28 and 29, wherein the lens 27 is a cementedlens consisting of a double-concave negative lens and a double-convexpositive lens, the lens 28 is a double-convex positive lens, and thelens 29 is a plano-convex positive lens.

Aspheric surfaces are used at the surface r₇ of the lens 21 on theprimary image-formation position side, both surfaces r₁₈ and r₁₉ of thepositive lens 25 and the surface r₂₁ of the condenser lens 26 on theprimary image-formation position side.

Note that in the numerical data given later, the relations of diopter tothe diopter control surface spaces d₂₃ and d₃₀ are shown, and the anglesof the optical axes reflected at the reflecting surfaces 13 b, 13 c andthe mirrors 14, 15 and 16 are indicated.

FIG. 23 is a section view of the finder optical system of NumericalEmbodiment 2 as taken apart along its optical axis, with its numericaldata given later. In FIG. 23, the optical members in the arrangement ofFIG. 15, too, are indicated by reference numerals. The relay opticalsystem is made up of lenses 21, 22, 23, 24, 25 and 26, wherein the lens21 is a positive meniscus lens convex on the primary image-formationposition side, the lens 22 is a positive meniscus lens convex on theprimary image-formation position side, the lens 23 is a double-concavenegative lens, the lens 24 is a cemented lens consisting of a negativemeniscus lens convex on the primary image-formation position side and adouble-convex positive lens, the lens 25 is a double-convex positivelens, and the lens 26 is a condenser lens consisting of a plano-convexpositive lens. The eyepiece optical system is made up of lenses 27, 28and 29, wherein the lens 27 is a cemented lens consisting of adouble-concave negative lens and a double-convex positive lens, the lens28 is a double-convex positive lens, and the lens 29 is a double-convexpositive lens.

In this embodiment, aspheric surfaces are used at both surfaces r₁₈ andr₁₉ of the positive lens 25, and the surface r₂₁ of the condenser lens26 on the primary image-formation position side.

Note that in the numerical data given later, the relations of diopter tothe diopter control surface spaces d₂₅ and d₃₀ are shown, and the anglesof the optical axes reflected at the reflecting surfaces 13 b, 13 c andthe mirrors 14, 15 and 16 are indicated.

In the numerical embodiments enumerated just below, the symbols usedhereinafter but not hereinbefore have the following meanings:

r₁, r₂, etc.: radius of curvature of each lens surface (opticalsurface),

d₁, d₂, etc.: space between adjacent lens surfaces (optical surfaces),

n_(d1), n_(d2), etc.: d-line refractive index of each lens (opticalmedium), and

v_(d1), v_(d2), etc.: Abbe constant of each lens (optical medium).

Here let x stand for an optical axis provided that the direction oftravel of light is taken as positive and y indicate a directionorthogonal to the optical axis. Then, aspheric shape is given by

x=(y ² /r)/[1+{1−(K+1)(y/r ²)^(1/2) ]+A ₄ y ⁴ +A ₆ y ⁶

where r is a paraxial radius of curvature, K is a conical coefficient,and A₄ and A₆ are the fourth- and the sixth-order coefficients,respectively.

Numerical embodiment: 1 (−1 diopter) r₁ = ∞ d₁ = 7.00 n_(d1) = 1.51633ν_(d1) = 64.14 (First image plane) d₂ = 10.00 n_(d2) = 1.51633 ν_(d2) =64.14 r₂ = ∞ d₃ = 22.80 n_(d3) = 1.51633 ν_(d3) = 64.14 r₃ = ∞ (F3) d₄ =10.80 n_(d4) = 1.69350 ν_(d4) = 53.21 r₄ = ∞ (F2) d₅ = 8.92 n_(d5) =1.80400 ν_(d5) = 46.57 r₅ = ∞ d₆ = 7.88 n_(d6) = 1.84666 ν_(d6) = 23.78r₆ = ∞ (F1) d₇ = 4.72 n_(d7) = 1.71736 ν_(d7) = 29.52 r₇ =14.94(Aspheric) d₈ = 0.44 n_(d8) = 1.80400 ν_(d8) = 46.57 r₈ = 200.43 d₉= 4.81 n_(d9) = 1.52542 ν_(d9) = 55.78 r₉ = 14.06 d₁₀ = 2.09 n_(d10) =1.52542 ν_(d10) = 55.78 r₁₀ = 25.38 d₁₁ = 0.75 n_(d11) = 1.51633 ν_(d11)= 64.14 r₁₁ = ∞ (Stop) d₁₂ = 1.10 n_(d12) = 1.84666 ν_(d12) = 23.78 r₁₂= −14.50 d₁₃ = 8.92 n_(d13) = 1.60311 ν_(d13) = 60.64 r₁₃ = 10.42 d₁₄ =1.60 n_(d14) = 1.78590 ν_(d14) = 44.20 r₁₄ = 300.05 d₁₅ = 5.46 n_(d15) =1.83400 ν_(d15) = 37.16 r₁₅ = 26.50 d₁₆ = 10.71 r₁₆ = −22.88 d₁₇ = 11.22r₁₇ = ∞ (R1) d₁₈ = 5.55 r₁₈ = 30.04(Aspheric) d₁₉ = 10.67 r₁₉ =276.45(Aspheric) d₂₀ = 11.00 r₂₀ = ∞ (R2) d₂₁ = 3.05 r₂₁ =56.07(Aspheric) d₂₂ = 0.00 r₂₂ = ∞ d₂₃ = 4.14 r₂₃ = ∞ d₂₄ = 1.00 (Secondimage plane) d₂₅ = 7.68 r₂₄ = ∞ d₂₆ = 1.37 r₂₅ = ∞ d₂₇ = 6.56 r₂₆ =−14.71 d₂₈ = 0.50 r₂₇ = 66.77 d₂₉ = 3.62 r₂₈ = −20.00 d₃₀ = 3.36 r₂₉ =244.69 d₃₁ = 3.30 r₃₀ = −39.79 d₃₂ = 23.00 r₃₁ = 37.55 r₃₂ = ∞ r₃₃ = ∞(Pupil of Observer) Aspherical Coefficients 7th surface K = 0.1155 A₄ =6.48 × 10⁻⁶ 18th surface K = −0.8404 A₄ = 1.39 × 10⁻⁵ 19th surface K =84.0521 A₄ = 1.29 × 10⁻⁵ 21th surface K = −0.6627 A₄ = 3.05 × 10⁻⁵Diopter Movement −1 diopter −3 diopter +1 diopter d₂₅ 7.68 5.50 9.84 d₃₀3.36 5.54 1.20 Reflection Angle of Optical Axis r₃ 106°  r₄ 116°  r₆ 78°r₁₇ 77° r₂₀ 75° α_(f) = 100° θ_(f) = 78° θ_(r) = 77° α_(r) = 75° θ_(m) =77.5°

Numerical embodiment: 2 (−1 diopter) r₁ = ∞ d₁ = 7.00 n_(d1) = 1.51633ν_(d1) = 64.14 (First image plane) d₂ = 10.00 n_(d2) = 1.51633 ν_(d2) =64.14 r₂ = ∞ d₃ = 22.80 n_(d3) = 1.51633 ν_(d3) = 64.14 r₃ = ∞ (F3) d₄ =10.80 n_(d4) = 1.71300 ν_(d4) = 53.87 r₄ = ∞ (F2) d₅ = 8.92 n_(d5) =1.80400 ν_(d5) = 46.57 r₅ = ∞ d₆ = 7.88 n_(d6) = 1.84666 ν_(d6) = 23.78r₆ = ∞ (F1) d₇ = 4.64 n_(d7) = 1.71736 ν_(d7) = 29.52 r₇ = 14.32 d₈ =0.41 n_(d8) = 1.80400 ν_(d8) = 46.57 r₈ = 484.25 d₉ = 4.78 n_(d9) =1.52542 ν_(d9) = 55.78 r₉ = 15.50 d₁₀ = 2.07 n_(d10) = 1.49236 ν_(d10) =57.86 r₁₀ = 23.20 d₁₁ = 0.75 n_(d11) = 1.51633 ν_(d11) = 64.14 r₁₁ = ∞(Stop) d₁₂ = 1.33 n_(d12) = 1.84666 ν_(d12) = 23.78 r₁₂ = −15.23 d₁₃ =8.86 n_(d13) = 1.60311 ν_(d13) = 60.64 r₁₃ = 10.30 d₁₄ = 1.49 n_(d14) =1.78590 ν_(d14) = 44.20 r₁₄ = 312.27 d₁₅ = 5.55 n_(d15) = 1.83400ν_(d15) = 37.16 r₁₅ = 27.57 d₁₆ = 10.71 r₁₆ = −22.16 d₁₇ = 11.24 r₁₇ = ∞(R1) d₁₈ = 5.53 r₁₈ = 31.04(Aspheric) d₁₉ = 10.67 r₁₉ =−291.42(Aspheric) d₂₀ = 11.00 r₂₀ = ∞ (R2) d₂₁ = 3.05 r₂₁ =52.58(Aspheric) d₂₂ = 0.00 r₂₂ = ∞ d₂₃ = 4.14 r₂₃ = ∞ d₂₄ = 1.00 (Secondimage plane) d₂₅ = 7.66 r₂₄ = ∞ d₂₆ = 1.34 r₂₅ = ∞ d₂₇ = 6.54 r₂₆ =−14.33 d₂₈ = 0.50 r₂₇ = 75.22 d₂₉ = 3.80 r₂₈ = −20.34 d₃₀ = 3.31 r₂₉ =238.56 d₃₁ = 3.23 r₃₀ = −37.80 d₃₂ = 23.00 r₃₁ = 40.09 r₃₂ = −797.23 r₃₃= ∞ (Pupil of Observer) Aspherical Coefficients 18th surface K = −0.8405A₄ = 1.73 × 10⁻⁵ 19th surface K = 84.0517 A₄ = 1.64 × 10⁻⁵ 21th surfaceK = −0.6627 A₄ = 4.50 × 10⁻⁵ A₆ = −1.81 × 10⁻⁷ Diopter Movement −1diopter −3 diopter +1 diopter d₂₅ 7.66 5.48 9.81 d₃₀ 3.31 5.49 1.16Reflection Angle of Optical Axis r₃ 106°  r₄ 116°  r₆ 78° r₁₇ 77° r₂₀75° α_(f) = 100° θ_(f) = 78° θ_(r) = 77° α_(r) = 75° θ_(m) = 77.5°

FIG. 24 is a collection of aberration diagrams for Embodiment 1, andFIG. 25 is a collection of aberration diagrams for Embodiment 2. Inthese aberration diagrams, (a), (b) and (c) are indicative of sphericalaberration (SA), astigmatism (AS), distortion (DT) and chromaticaberration of magnification (CC) at the time of +1 diopter, −1 diopter,and −3 diopter, respectively, and “φ” and “ω” are indicative of pupildiameter and an exit angle, respectively. Note that the states ofaberrations in the aberration diagrams are illustrative of those in thefinder optical system after the primary image-formation plane.

The angle of reflection of the optical axis contemplated in thearrangement of FIG. 20 is typically 68° for the quick return mirror 61,78° for the mirror 63, 48° for the mirror 64, and 58° for the mirror 65.

The finder optical system and single-lens reflex camera according to thethird aspect of the invention are now explained with reference to someembodiments.

FIG. 26 is a sectional view of the finder optical system of Embodiment 1according to the third aspect of the invention, as taken apart along itsoptical axis. This numerical embodiment is substantially the same as thefinder optical system of Numerical Embodiment 1 according to the secondaspect of the invention.

Referring to FIG. 26, the finder optical system is made up of a lensgroup having positive refracting power and consisting of a first lens ofpositive refracting power indicated at surfaces r₇-r₈, a second lens ofpositive refracting power indicated at surfaces r₉-r₁₀, a third lens ofnegative refracting power indicated at surfaces r₁₂-r₁₃, and a cementeddoublet consisting of a fourth lens of negative refracting power and afifth lens of positive refracting power, indicated at surfaces r₁₄-r₁₆,a relay optical system auxiliary lens of positive refracting powerindicated at surfaces r₁₈-r₁₉, and a condenser lens of positiverefracting power indicated at surfaces r₂₁-r₂₂, wherein a subject imageformed as a primary image at the primary image-formation positionindicated at surface r₁ is re-formed at the secondary image-formationposition indicated at surface r₂₃, and an aperture stop indicated atsurface r₁₁ is located between the second lens and the third lens. Inthis embodiment, the secondary image-formation position defined bysurface r₂₃ is in alignment with the surface r₂₂ of the condenser lens26 that faces away the primary image-formation side.

The image re-formed at the secondary image-formation position r₂₃ isenlarged and viewed through the pupil of the viewer positioned at thesurface r₃₃, i.e., the eye point EP via an eyepiece optical system madeup of a cemented lens consisting of a double-concave negative lens and adouble-convex positive lens and indicated at surfaces r₂₆-r₂₈, adouble-convex positive lens indicated at surfaces r₂₉-r₃₀ and aplano-convex positive lens indicated at surfaces r₃₁-r₃₂.

In the arrangement of FIG. 26, note that the surfaces r₂-r₅ between theprimary image-formation position and the first lens could be thought ofas a prism 13 (referred to later) for bending the optical axis, thesurface r₆ as a mirror 14 (referred to later) for bending the opticalaxis, the surface r₁₇ between the fifth lens and the relay opticalsystem auxiliary lens as a mirror 15 (referred to later) for bending theoptical axis, and the surface r₂₀ between the relay optical systemauxiliary lens and the condenser lens as a mirror 16 (referred to later)for bending the optical axis. Also note that a plane-parallel plateindicated at surfaces r₂₄-r₂₅ between the secondary image-formationposition and the eyepiece optical system could be thought of as thedust-preventive glass 32 to be referred to later.

Although numerical data in this embodiment are the same as those inNumerical Embodiment 1 according to the second aspect of the invention,it is understood that in the finder optical system, the first lens is apositive meniscus lens convex on the primary image-formation positionside, the second lens is a positive meniscus lens convex on the primaryimage-formation position side, the third lens is a double-concavenegative lens, the fourth lens is a negative meniscus lens convex on theprimary image-formation position side, and the fifth lens is adouble-convex positive lens. The relay optical system auxiliary lens isa positive meniscus lens convex on the primary image-formation positionside, and the condenser lens is a plano-convex positive lens.

Aspheric surfaces are used at the surface r₇ of the first lens on theprimary image-formation position side, both surfaces r₁₈ and r₁₉ of therelay optical system auxiliary lens and the surface r₂₁ of the condenserlens on the primary image-formation position side.

In the numerical data about Numerical Embodiment 1, the relations ofdiopter to the diopter control surface spaces d₂₅ and d₃₀ are alsoshown.

FIG. 27 is a sectional view of the finder optical system of Embodiment 2according to the third aspect of the invention, as taken apart along itsoptical axis. This finder optical system is substantially the same asthat of Numerical Embodiment 2 according to the second aspect of FIG.23.

Basically, this embodiment is much the same as in Embodiment 1 of FIG.26, except that the relay optical system auxiliary lens is adouble-convex positive, the lens located in, and nearest to the eyepoint side of, the eyepiece optical system is a double-convex positivelens, and three aspheric surfaces are used: two at both surfaces r₁₈ andr₁₉ of the relay optical system auxiliary lens and one at the surfacer₂₁ of the condenser lens on the primary image-formation position side.

Aberration diagrams for the above Embodiments 1 and 2 are the same as inFIGS. 24 and 25, respectively.

Tabulated below are the values of conditions (3-1), (3-2), (3-3) and(3-4) in the above Embodiments 1 and 2.

Condition (3-1) (3-2) (3-3) (3-4) dl/flr drf/drr ds/flr dh/ds Embodiment1 1.66 0.44 1.29 0.42 Embodiment 2 1.65 0.47 1.28 0.42

Reference is now made to some embodiments of the inventive-single-lensreflex camera to which such an exemplified finder optical system isapplied. FIG. 28 is illustrative in section of the construction of asingle-lens reflex camera that incorporates the finder optical system ofEmbodiment 1 of according to the third aspect of the invention, as inFIG. 15. Referring now to FIG. 28, reference numeral 1 stands for asingle-lens reflex camera that incorporates the finder optical systemaccording to that embodiment of the invention, and reference numeral 2indicates an interchangeable taking lens. Note here that the taking lens2 could be made integral with a camera body.

Reference is now made with reference to the order of travel of a lightbeam emanating from a subject and exiting the taking lens 2.

A light beam emanating from the taking lens 2 is reflected at a quickreturn mirror 11 at an angle of reflection of 90° in this embodiment.Note here that when a half-silvered mirror is used as the quick returnmirror 11, the light beam transmitting through it could be guided to afocal detection means (not shown).

In what follows, on the basis of the position where the optical axisexiting from the taking lens 2 is reflected at the quick return mirror11, a direction which is vertical to the optical axis exiting from thetaking lens 2 and in which the optical axis is reflected at the quickreturn mirror 11 will be referred to as an upward direction, a directionwhich is parallel with the optical axis of the taking lens 2 and inwhich the taking lens 2 is located will be called a subject direction,and a direction reverse to the direction of locating the taking lens 2will be called a viewer direction.

The light beam reflected at the quick return mirror 11 is incident on afocal plane plate 12 placed at a position optically equivalent(conjugate) to an image pickup device 103 to be described later.Referring here to FIG. 16 to be described later, when a subject image isformed on the image pickup device 103, it is also formed on the focalplane plate 12. The focal plane plate 12 could also have a condenserlens function.

The light beam exiting from the focal plane plate 12 is incident on aprism 13. The prism 13 has an entrance surface 13 a, a reflectingsurface 13 b, a reflecting surface 13 c and an exit surface 13 d.Desirously, the entrance surface 13 a is vertical to the axis ofincident light. The light beam incident on the entrance surface 13 a isreflected at the reflecting surface 13 b in the subject direction. Lightquantity losses here can be reduced by the satisfaction of thetotal-reflection condition. Further, the light beam is reflected in theupward direction such that the optical axis has components in the upwarddirection and subject direction at the reflecting surface 13 c (that is,the optical axis direction lies between the subject direction and theupward direction). Light quantity losses here can be reduced by thesatisfaction of the total-reflection condition. Further, the light beamleaves the prism 13 through the exit surface 13 d. The exit surface 13 dhere is desirously vertical to the optical axis.

The light beam exiting the prism 13 is further reflected at a mirror 14in the viewer direction, with the optical axis substantially parallelwith the optical axis of the taking lens 2.

Then, the light beam transmits through lens 21, lens 22, lens 23 andlens 24 that form together the relay optical system where it issubjected to lens actions. In the embodiment of FIG. 28, the lens 21 isa positive meniscus lens tightly convex on the subject side or the firstlens in Embodiment 1, and the lens 22 is a positive meniscus lenstightly convex on the subject side or the second lens in Embodiment 1;two such lenses are tantamount to the FP lens group. The lens 23 is adouble-concave negative lens or the third lens in Embodiment 1,tantamount to the N lens group. The lens 24 is a cemented doubletcomposed of a negative meniscus lens convex on the subject side and adouble-convex positive lens and having a positive composite power,wherein the negative meniscus lens is the fourth lens, and thedouble-convex positive lens is the fifth lens in Embodiment 1. Thiscemented doublet is tantamount to the RP lens group. The lens groupslocated here are located such that their composite power turns positive,representing a substantial part of the relay optical system.

Note that an aperture stop 31 could be located near the negative lens 23for efficient pupil transmission.

Next, the light beam is reflected at a mirror 15 to bend the opticalaxis in the subject direction and a downward direction, and the opticalaxis is reflected at a mirror 16 in the viewer direction. Here, apositive lens 25 is located between the mirrors 15 and 16, therebyimproving on the image-formation capability and pupil transmissioncapability of the relay optical system. The positive lens 25 istantamount to the relay optical system auxiliary lens.

The light beam reflected at the mirror 16 is incident on a condenserlens 26 placed near the secondary image-formation position 17. In theembodiment of FIG. 28, the condenser lens 26 is a plano-convex positivelens convex on the subject side, with its plane side substantially inalignment with the secondary image-formation position 17.

Further, the light beam passes through a dust-preventive glass 32. Thedust-preventive glass 32 cooperates with other frame or the like toprevent dusts and other contaminants from deposition onto lens surfacesnear the secondary image-formation position 17.

Further, the light beam is subjected to lens actions at lens 27, lens 28and lens 29 forming together the eyepiece optical system, exiting from acamera body. Finally, the light beam is guided to the eye of the viewer.

Note that affixed to FIGS. 26 and 27 are reference numerals indicativeof elements corresponding to those in FIG. 28.

FIG. 29 is illustrative of in what state the single-lens reflex cameraof FIG. 28 is operated. Note however that the outline indicative of thecamera body is not drawn.

Upon operation, the quick return mirror 11 is retracted back from theoptical path, and a light beam leaving the taking lens 2 transmitssuccessively through a filter 101 and a filter 102, entering the imagepickup device 103. The filers 101 and 102 are each imparted with somefunctions such as an infrared cut filter function, a low-pass filterfunction and a dust-preventive filter function, and their number is notnecessarily limited to two. The image pickup device 103 is an electronicimage pickup device such as CCS or C-MOS, or a silver halide film.

In the embodiment using the inventive finder optical system, it isunderstood that misalignments in the conjugate relation between theprimary image-formation position and the second image-formationposition, if any, could be corrected by control of one or two spaces inthe relay optical system.

When two spaces are used, that control could be implemented by axialmovement of a part of the relay optical system in such a way as toreduce the sum of control amount down to zero.

Specifically, it is desired to control any one of the following sites.In Numerical Embodiments 1 and 2, for instance, it is desired to controla space d₆ between the primary image-formation position r₁ and the lens21 (the first lens), a space d₁₀+d₁₁ between the lens 22 (the secondlens) and the lens 23 (the third lens), movement of the lens 23 (thethird lens) alone, movement of the lens 24 (the fourth+the fifth lens)alone, and a space between the lens 25 (the relay optical systemauxiliary lens) and the lens 26 (the condenser lens).

To allow the optical axis of the taking lens 2 to have a given relationto the optical axis of the eyepiece system made up of lenses 27-29, thepositions of the mirrors 14-16 located in the relay optical system couldbe controlled and corrected.

In the invention, a lens function surface could be located near theprimary image-formation plane or between the primary image-formationplane and the FP lens group (the first lens+the second lens) toimplement condenser or other functions or, alternatively, a lensfunction surface could be interposed between the RP lens group (thefourth lens+the fifth lens) and the eyepiece lens group (the lenses 27,28 and 29) to implement condenser or other functions.

The eyepiece optical system and relay type finder optical systemaccording to the fourth aspect of the invention, and a single-lensreflex camera that incorporates them are now explained with reference totheir embodiments.

FIG. 30 is a sectional view of the finder optical system of Embodiment 1according to the fourth aspect of the invention, as taken apart alongits optical axis. This embodiment is substantially the same in numericaldata as the finder optical system of Numerical Embodiment 1 according tothe second aspect of the invention.

Referring to FIG. 30, the relay optical system in this finder opticalsystem is made up of a first lens of positive refracting power indicatedat surfaces r₇-r₈, a second lens of positive refracting power indicatedat surfaces r₉-r₁₀, a third lens of negative refracting power indicatedat surfaces r₁₂-r₁₃, a cemented doublet of positive refracting powercomposed of a fourth lens of negative refracting power and a fifth lensof positive refracting power and indicated at surfaces r₁₄-r₁₆, a sixlens of positive refracting power indicated at surfaces r₁₈-r₁₉, and acondenser lens of positive refracting power indicated at surfacer₂₁-r₂₂, and is operable to re-form a subject image at the secondaryimage-formation position indicated at surface r₂₃, wherein the subjectimage is formed as a primary image on the primary image-formationposition indicated at surface r₁. Between the second lens and the thirdlens, there is an aperture stop indicated at surface r₁₁. In thisembodiment, the secondary image-formation position indicated at surfacer₂₃ is in alignment with the surface r₂₂ of the condenser lens 26 thatfaces away the primary image-formation side.

The image re-formed at the secondary image-formation position r₂₃ ismagnified and viewed through the pupil of the viewer positioned atsurface r₃₃, viz., the eye point EP via the eyepiece optical system madeup of a cemented doublet of a first lens consisting of a double-concavenegative lens and a second lens consisting of a double-convex positivelens and indicated at surfaces r₂₆-r₂₈, a third lens consisting of adouble-convex positive lens indicated at surfaces r₂₉-r₃₀, and afourth-lens consisting of a plano-convex positive lens indicated atsurfaces r₃₁-r₃₂.

In the arrangement of FIG. 30, the surfaces r₂-r₅ between the primaryimage-formation position and the first lens in the relay optical systemcould be thought of as a prism 13 (described later) for bending theoptical axis, the surface r₆ could be thought of as a mirror 14(described later) for bending the optical axis, the surface r₁₇ betweenthe fifth lens and the sixth lens forming together a part of the relayoptical system could be thought of as a mirror 15 (described later) forbending the optical axis, and the surface r₂₀ between the sixth lens andthe condenser lens forming together a part of the relay optical systemcould be thought of as a mirror 16 (described later) for bending theoptical axis. A plane-parallel plate located between the secondaryimage-formation position and the eyepiece optical system and indicatedat surfaces r₂₄-r₂₅ could be thought of as the dust-preventive glass 32to be described later.

Numerical data on this embodiment are the same as those on NumericalEmbodiment 1 according to the second aspect of the invention. However,the first, the second, the third, the fourth, the fifth, the sixth, andthe condenser lens that form together the finder optical system are apositive meniscus lens convex on the primary image-formation positionside, a positive meniscus lens convex on the primary image-formationposition side, a double-concave negative lens, a negative meniscus lensconvex on the primary image-formation position side, a double-convexpositive lens, a positive meniscus lens convex on the primaryimage-formation position side, and a plano-convex positive lens,respectively.

Aspheric surfaces are used at the surface r₇ of the first lens in thefinder optical system on the primary image-formation position side, bothsurfaces r₁₈ and r₁₉ of the sixth lens, and the surface r₂₁ of thecondenser lens on the primary image-formation position side.

Note that in the numerical data in Numerical Embodiment 1, the relationsof diopter to the diopter control surface spaces d₂₅ and d₃₀ are shown.

FIG. 31 is a sectional view of the finder optical system of Embodiment 2according to the fourth aspect of the invention, as taken apart alongits optical axis. This embodiment is substantially as the same innumerical data as the finder optical system of Numerical Embodiment 2according to the second aspect of the invention shown in FIG. 23.

Basically, this embodiment is much the same as that of FIG. 30 exceptthat the sixth lens in the finder optical system is a double-convexpositive lens, the fourth lens in the eyepiece optical system is adouble-convex positive lens, and three aspheric surfaces are used: twoat both surfaces r₁₈ and r₁₉ of the sixth lens in the finder opticalsystem and one at the surface r₂₁ of the condenser lens on the primaryimage-formation position side.

FIGS. 32 and 33 are aberration diagrams for only the eyepiece opticalsystems in the above Embodiments 1 and 2, respectively. Note that theaberration diagrams for the whole finder optical systems in Embodiments1 and 2 are the same as in FIGS. 24 and 25, respectively.

In the aberration diagrams of FIGS. 32 and 33, the maximum ray height atthe secondary image-formation plane is set at 9.56 mm, and 9.55 mm,respectively, and in the aberration diagrams of FIGS. 24 and 25, themaximum ray height at the primary image-formation plane is set at 11.15mm. The maximum exit angle in FIGS. 24 and 25 is smaller than that inFIGS. 32 and 33, because presentation of information, etc. around thesecondary image-formation plane is taken into consideration.

Tabulated below are the value of conditions (4-1), (4-2), (4-3) and(4-4) in Embodiments 1 and 2 above.

Condition (4-1) (4-2) (4-3) (4-4) f123/fA f4/fA d4/fA fA Embodiment 14.50 1.69 0.12 26.66 Embodiment 2 4.42 1.71 0.12 26.82

Reference is now made to one exemplary single-lens reflex camera towhich such a finder optical system as exemplified above is appliedaccording to the invention. FIG. 34 is a sectional view of thearrangement of the single-lens reflex camera that incorporates thefinder optical system as exemplified above. In FIG. 34, referencenumeral 1 is a single-lens reflex camera that incorporates that finderoptical system, and 2 is an interchangeable taking lens. Note that thetaking lens 2 could be integral with a camera body.

Reference is now made with reference to the order of travel of a lightbeam emanating from a subject and exiting the taking lens 2.

A light beam emanating from the taking lens 2 is reflected at a quickreturn mirror 11 at an angle of reflection of 90° in this embodiment.Note here that when a half-silvered mirror is used as the quick returnmirror 11, the light beam transmitting through it could be guided to afocal detection means (not shown).

In what follows, on the basis of the position where the optical axisexiting from the taking lens 2 is reflected at the quick return mirror11, a direction which is vertical to the optical axis exiting from thetaking lens 2 and in which the optical axis is reflected at the quickreturn mirror 11 will be referred to as an upward direction, a directionwhich is parallel with the optical axis of the taking lens 2 and inwhich the taking lens 2 is located will be called a subject direction,and a direction reverse to the direction of locating the taking lens 2will be called a viewer direction.

The light beam reflected at the quick return mirror 11 is incident on afocal plane plate 12 placed at a position optically equivalent(conjugate) to an image pickup device 103 to be described later.Referring here to FIG. 35 to be mentioned later, when a subject image isformed on the image pickup device 103, it is also formed on the focalplane plate 12. The focal plane plate 12 could also have a condenserlens function.

The light beam exiting the focal plane plate 12 is incident on a prism13. The prism 13 has an entrance surface 13 a, a reflecting surface 13b, a reflecting surface 13 c and an exit surface 13 d. Desirously, theentrance surface 13 a is vertical to the axis of incident light. Thelight beam incident on the entrance surface 13 a is reflected at thereflecting surface 13 b in the subject direction. Light quantity losseshere can be reduced by the satisfaction of the total-reflectioncondition. Further, the light beam is reflected in the upward directionsuch that the optical axis has components in the upward direction andsubject direction at the reflecting surface 13 c (that is, the opticalaxis direction lies between the subject direction and the upwarddirection). Light quantity losses here can be reduced by thesatisfaction of the total-reflection condition. Further, the light beamleaves the prism 13 through the exit surface 13 d. The exit surface 13 dhere is desirously vertical to the optical axis.

The light beam exiting the prism 13 is further reflected at a mirror 14in the viewer direction, with the optical axis roughly parallel with theoptical axis of the taking lens 2.

Then, the light beam transmits through lens 21, lens 22, lens 23 andlens 24 that form together the relay optical system where it issubjected to lens actions. In the embodiment of FIG. 34, the lens 21 isa positive meniscus lens tightly convex on the subject side, the lens 22is a positive meniscus lens tightly convex on the subject side, the lens23 is a double-concave negative lens, and the lens 24 is a cementeddoublet composed of a negative meniscus lens convex on the subject sideand a double-convex positive lens and having a positive composite power.

The lens groups located here are constructed such that their compositepower turns positive, and required to represent a substantial part ofthe relay optical system. A specific lens arrangement should preferablybe designed while taking aberrations, etc. into consideration. Note thatan aperture stop 31 could be located near the negative lens 23 forefficient pupil transmission.

Next, the light beam is reflected at a mirror 15 to bend the opticalaxis in the subject direction and a downward direction, and the opticalaxis is reflected at a mirror 16 in the viewer direction. Here, apositive lens 25 is located between the mirrors 15 and 16, therebyimproving on the image-formation capability and pupil transmissioncapability of the relay optical system.

The light beam reflected at the mirror 16 is incident on a condenserlens 26 placed near the secondary image-formation position 17. In theembodiment of FIG. 34, the condenser lens 26 is a plano-convex positivelens convex on the subject side, with its plane side substantially inalignment with the secondary image-formation position 17.

Further, the light beam passes through a dust-preventive glass 32. Thedust-preventive glass 32 cooperates with other frame or the like toprevent dusts and other contaminants from deposition onto lens surfacesnear the secondary image-formation position 17.

Further, the light beam is subjected to lens actions at lens 27, lens 28and lens 29 forming together the eyepiece optical system, exiting from acamera body. Finally, the light beam is guided to the eye of the viewer.

Note that the lens 27 is tantamount to the cemented doublet of the firstlens and the second lens in the eyepiece optical system of theinvention. Specifically in Embodiment 1 of FIG. 34, the cemented doubletconsists of a double-concave negative lens and a double-convex positivelens. The lens 28 and the lens 29 are tantamount to the third lens andthe fourth lens in the eyepiece optical system of the invention,respectively. Referring specifically to Embodiment 1 of FIG. 34, thelens 28 is a double-convex positive lens, and the lens 29 is aplano-convex positive lens.

FIG. 35 is illustrative of in what state the single-lens reflex cameraof FIG. 34 is operated. Note however that the outline indicative of thecamera body is not drawn.

Upon operation, the quick return mirror 11 is retracted back from theoptical path, and a light beam leaving the taking lens 2 transmitssuccessively through a filter 101 and a filter 102, entering the imagepickup device 103. The filers 101 and 102 are each imparted with somefunctions such as an infrared cut filter function, a low-pass filterfunction and a dust-preventive filter function, and their number is notnecessarily limited to two. The image pickup device 103 is an electronicimage pickup device such as CCS or C-MOS, or a silver halide film.

In the embodiment using the inventive finder optical system, it isunderstood that misalignments in the conjugate relation between theprimary image-formation position and the second image-formationposition, if any, could be corrected by control of one or two spaces inthe relay optical system.

When two spaces are used, that control could be implemented by axialmovement of a part of the relay optical system in such a way as toreduce the sum of control amount down to zero.

To allow the optical axis of the taking lens 2 to have a given relationto the optical axis of the eyepiece system made up of lenses 27-29, thepositions of the mirrors 14-16 located in the relay optical system couldbe controlled and corrected.

Alternatively, a part or the whole of the eyepiece optical system couldbe moved in correspondence to the viewer's diopter.

In the invention, a lens function surface could be located near thesecondary image-formation plane or between the secondary image-formationplane and the lens group in the eyepiece optical system to implementcondenser or other functions.

The relay type finder optical system, and the single-lens reflex camera,according to the fifth aspect of the invention is now explained withreference to one specific embodiment.

FIG. 36 is a sectional view of the arrangement of a single-lens reflexcamera on which the relay type finder optical system according to oneembodiment of the fourth aspect of the invention. In FIG. 36, referencenumeral 1 is a single-lens reflex camera that incorporates that relaytype finder optical system, and 2 is an interchangeable taking lens.Note that the taking lens 2 could be integral with a camera body.

Reference is now made with reference to the order of travel of a lightbeam emanating from a subject and exiting the taking lens 2.

A light beam emanating from the taking lens 2 is reflected at a quickreturn mirror 11 at an angle of reflection of 90° in this embodiment.Note here that when a half-silvered mirror is used as the quick returnmirror 11, the light beam transmitting through it could be guided to afocal detection means (not shown).

In what follows, on the basis of the position where the optical axisexiting from the taking lens 2 is reflected at the quick return mirror11, a direction which is vertical to the optical axis exiting from thetaking lens 2 and in which the optical axis is reflected at the quickreturn mirror 11 will be referred to as an upward direction, a directionwhich is parallel with the optical axis of the taking lens 2 and inwhich the taking lens 2 is located will be called a subject direction,and a direction reverse to the direction of locating the taking lens 2will be called a viewer direction.

The light beam reflected at the quick return mirror 11 is incident on afocal plane plate 12 placed at a position optically equivalent(conjugate) to an image pickup device 103 to be described later.Referring here to FIG. 37 to be mentioned later, when a subject image isformed on the image pickup device 103, it is also formed on the focalplane plate 12. The focal plane plate 12 could also have a condenserlens function.

The light beam exiting the focal plane plate 12 is incident on a prism13. Corresponding to the aforesaid prism P1, the prism 13 has anentrance surface 13 a, a reflecting surface 13 b, a reflecting surface13 c and an exit surface 13 d. Desirously, the entrance surface 13 a isvertical to the incident optical axis. The light beam incident on theentrance surface 13 a is reflected at the reflecting surface 13 b in thesubject direction. Light quantity losses here can be reduced by thesatisfaction of the total-reflection condition. Further, the light beamis reflected in the upward direction such that the optical axis hascomponents in the upward direction and subject direction at thereflecting surface 13 c (that is, the optical axis direction liesbetween the subject direction and the upward direction). Light quantitylosses here can be reduced by the satisfaction of the total-reflectioncondition. Note that the reflecting surfaces 13 b and 13 c aretantamount to the aforesaid set PM of back-to-back two reflectingsurfaces. Further, the light beam leaves the prism 13 through the exitsurface 13 d. The exit surface 13 d here is desirously vertical to theoptical axis.

The light beam exiting the prism 13 is further reflected at a mirror 14in the viewer direction, with the optical axis roughly parallel with theoptical axis of the taking lens 2.

Then, the light beam transmits through lens 21, lens 22, lens 23 andlens 24 that form together the relay optical system where it issubjected to lens actions. In Embodiment 1 of FIG. 36 (corresponding toNumerical Embodiment 2), the lens 21 is a positive meniscus lens tightlyconvex on the subject side, the lens 22 is a positive meniscus lenstightly convex on the subject side, the lens 23 is a double-concavenegative lens, and the lens 24 is a cemented doublet composed of anegative meniscus lens convex on the subject side and a double-convexpositive lens and having a positive composite power.

The lens groups located here are constructed such that their compositepower turns positive, and required to represent a substantial part ofthe relay optical system. A specific lens arrangement should preferablybe designed while taking aberrations, etc. into consideration.

It is then preferable that at least one positive lens (tantamount to thelens 21 or the lens 22 in Embodiment 1 of FIG. 36) is provided, at leastone negative lens (tantamount to the lens 23 in Embodiment 1 of FIG. 36)is located on the side of the positive lens facing the secondaryimage-formation position, and at least one positive lens (tantamount tothe positive lens 24 in Embodiment 1 of FIG. 36) is located on the sideof the negative lens facing the secondary image-formation position,because the principal points are set in this region so thatmagnification and optical performance are easily ensured. Note that anaperture stop 31 could be positioned near that negative lens forefficient pupil transmission.

Next, the light beam is reflected at a mirror 15 to bend the opticalaxis in the subject direction and a downward direction, and the opticalaxis is reflected at a mirror 16 in the viewer direction. Here, apositive lens 25 is located between the mirrors 15 and 16, therebyimproving on the image-formation capability and pupil transmissioncapability of the relay optical system.

The light beam reflected at the mirror 16 is incident on a condenserlens 26 placed near the secondary image-formation position 17. InEmbodiment 1 of FIG. 36, the condenser lens 26 is a plano-convexpositive lens convex on the subject side, with its plane sidesubstantially in alignment with the secondary image-formation position17.

Further, the light beam passes through a dust-preventive glass 32. Thedust-preventive glass 32 cooperates with other frame or the like toprevent dusts and other contaminants from deposition onto lens surfacesnear the secondary image-formation position 17.

Further, the light beam is subjected to lens actions at lens 27, lens 28and lens 29 forming together the eyepiece optical system, exiting from acamera body. Finally, the light beam is guided to the eye of the viewer.

FIG. 37 is illustrative of in what state the single-lens reflex cameraof FIG. 36 is operated. Note however that the outline indicative of thecamera body is not drawn.

Upon operation, the quick return mirror 11 is retracted back from theoptical path, and a light beam leaving the taking lens 2 transmitssuccessively through a filter 101 and a filter 102, entering the imagepickup device 103. The filers 101 and 102 are each imparted with somefunctions such as an infrared cut filter function, a low-pass filterfunction and a dust-preventive filter function, and their number is notnecessarily limited to two. The image pickup device 103 is an electronicimage pickup device such as CCS or C-MOS, or a silver halide film.

FIG. 38 is a schematic representation of an arrangement wherein thequick return mirror of FIG. 36 is replaced by a prism group 41 having ahalf-silvered mirror plane 41 a. Otherwise, this arrangement is the sameas in FIG. 36, and so will not be set forth any longer (especiallyregarding the lens system). With this arrangement, a subject could bephotographed at the same as it is viewed.

FIG. 39 is illustrative of a modification to the arrangement of FIG. 36,wherein a lens function is imparted to a prism 13 tantamount to theprism P1. In FIG. 39, reference numeral 1 is a single-lens reflex camerathat incorporates the relay type finder optical system embodied herein,and 2 is an interchangeable taking lens. Note that the taking lens couldbe integral with a camera body.

Reference is now made with reference to the order of travel of a lightbeam emanating from a subject and exiting the taking lens 2.

A light beam emanating from the taking lens 2 is reflected at a quickreturn mirror 11 at an angle of reflection of 90° in this embodiment.Note here that when a half-silvered mirror is used as the quick returnmirror 11, the light beam transmitting through it could be guided to afocal detection means (not shown).

In what follows, on the basis of the position where the optical axisexiting from the taking lens 2 is reflected at the quick return mirror11, a direction which is vertical to the optical axis exiting from thetaking lens 2 and in which the optical axis is reflected at the quickreturn mirror 11 will be referred to as an upward direction, a directionwhich is parallel with the optical axis of the taking lens 2 and inwhich the taking lens 2 is located will be called a subject direction,and a direction reverse to the direction of locating the taking lens 2will be called a viewer direction.

The light beam reflected at the quick return mirror 11 is incident on afocal plane plate 12 placed at a position optically equivalent(conjugate) to an image pickup device 103 to be described later.Referring here to FIG. 37 to be mentioned later, when a subject image isformed on the image pickup device 103, it is also formed on the focalplane plate 12. The focal plane plate 12 could also have a condenserlens function.

The light beam exiting the focal plane plate 12 is incident on a prism13. Corresponding to the aforesaid prism P1, the prism 13 has anentrance surface 13 a, a reflecting surface 13 b, a reflecting surface13 c and an exit surface 13 d. Desirously, the entrance surface 13 a isvertical to the incident optical axis. The light beam incident on theentrance surface 13 a is reflected at the reflecting surface 13 b in thesubject direction. Light quantity losses here can be reduced by thesatisfaction of the total-reflection condition. Further, the light beamis reflected in the upward direction such that the optical axis hascomponents in the upward direction and subject direction at thereflecting surface 13 c (that is, the optical axis direction liesbetween the subject direction and the upward direction). Light quantitylosses here can be reduced by the satisfaction of the total-reflectioncondition. Note that the reflecting surfaces 13 b and 13 c aretantamount to the aforesaid set PM of back-to-back two reflectingsurfaces. Further, the light beam leaves the prism 13 through the exitsurface 13 d. The exit surface 13 d here is in a convex lens form, andshares a part of the function of the relay optical system.

The light beam exiting the prism 13 is further reflected at a mirror 14in the viewer direction, with the optical axis roughly parallel with theoptical axis of the taking lens 2.

Then, the light beam transmits through lens 21, lens 22, lens 23 andlens 24 that form together the relay optical system where it issubjected to lens actions. In the embodiment of FIG. 39, the lens 21 isa positive meniscus lens tightly convex on the subject side, the lens 22is a positive meniscus lens tightly convex on the subject side, the lens23 is a double-concave negative lens, and the lens 24 is a cementeddoublet composed of a negative meniscus lens convex on the subject sideand a double-convex positive lens and having a positive composite power.

The lens groups located here are constructed such that their compositepower turns positive, and required to represent a substantial part ofthe relay optical system. A specific lens arrangement should preferablybe designed while taking aberrations, etc. into consideration.

It is then preferable that at least one positive lens (tantamount to thelens 21 or the lens 22 in the embodiment of FIG. 39) is provided, atleast one negative lens (tantamount to the lens 23 in the embodiment ofFIG. 39) is located on the side of the positive lens facing thesecondary image-formation position, and at least one positive lens(tantamount to the positive lens 24 in the embodiment of FIG. 39) islocated on the side of the negative lens facing the secondaryimage-formation position, because the principal points are set in thisregion so that magnification and optical performance are easily ensured.Note that an aperture stop 31 could be positioned near that negativelens for efficient pupil transmission.

Next, the light beam is reflected at a mirror 15 to bend the opticalaxis in the subject direction and a downward direction, and the opticalaxis is reflected at a mirror 16 in the viewer direction. Here, apositive lens 25 is located between the mirrors 15 and 16, therebyimproving on the image-formation capability and pupil transmissioncapability of the relay optical system.

The light beam reflected at the mirror 16 is incident on a condenserlens 26 placed near the secondary image-formation position 17. In theembodiment of FIG. 39, the condenser lens 26 is a plano-convex positivelens convex on the subject side, with its plane side substantially inalignment with the secondary image-formation position 17.

Further, the light beam passes through a dust-preventive glass 32. Thedust-preventive glass 32 cooperates with other frame or the like toprevent dusts and other contaminants from deposition onto lens surfacesnear the secondary image-formation position 17.

Further, the light beam is subjected to lens actions at lens 27, lens 28and lens 29 forming together the eyepiece optical system, exiting from acamera body. Finally, the light beam is guided to the eye of the viewer.

FIG. 40 is illustrative, as in FIG. 36, of an embodiment wherein theaforesaid prism P1 is located between the aforesaid positive lens groupGL and the secondary image-formation position. In FIG. 39, referencenumeral 1 is a single-lens reflex camera that incorporates the relaytype finder optical system embodied herein, and 2 is an interchangeabletaking lens. Note that the taking lens could be integral with a camerabody.

Reference is now made with reference to the order of travel of a lightbeam emanating from a subject and exiting the taking lens 2.

A light beam emanating from the taking lens 2 is reflected at a quickreturn mirror 11 at an angle of reflection of 90° in this embodiment.Note here that when a half-silvered mirror is used as the quick returnmirror 11, the light beam transmitting through it could be guided to afocal detection means (not shown).

In what follows, on the basis of the position where the optical axisexiting from the taking lens 2 is reflected at the quick return mirror11, a direction which is vertical to the optical axis exiting from thetaking lens 2 and in which the optical axis is reflected at the quickreturn mirror 11 will be referred to as an upward direction, a directionwhich is parallel with the optical axis of the taking lens 2 and inwhich the taking lens 2 is located will be called a subject direction,and a direction reverse to the direction of locating the taking lens 2will be called a viewer direction.

The light beam reflected at the quick return mirror 11 is incident on afocal plane plate 12 placed at a position optically equivalent(conjugate) to an image pickup device 103 to be described later.Referring here to FIG. 37 to be mentioned later, when a subject image isformed on the image pickup device 103, it is also formed on the focalplane plate 12. The focal plane plate 12 could also have a condenserlens function.

The light beam exiting the focal plane plate 12 has an optical axisacutely reflected at a mirror 51 in the subject direction, and is thenincident on a positive lens 71 that is intended to improve on theimage-formation capability or the pupil transmission capability of therelay optical system.

Further, the optical axis is acutely reflected at a mirror 52 in theupward direction, and then subjected to lens actions at lenses 72, 73,74 and 75 forming together the relay optical system. The lens 72 is acemented doublet composed of a positive lens and a negative lens andhaving generally positive power, the lens 73 is a negative lens, thelens 74 is a positive lens, and the lens 75 is a positive lens. Thelenses 72-75 are tantamount to the aforesaid positive lens group RL.Note that an aperture stop 31 could be located near the negative lens 73for efficient pupil transmission.

Further, the light beam is reflected at a mirror 53 in the viewerdirection for incidence on a prism 54 tantamount the aforesaid prism P1.The prism 54 has an entrance surface 54 a, a reflecting surface 54 b, areflecting surface 54 c and an exit surface 54 d.

Desirously, the entrance surface 54 a is vertical to the incidentoptical axis. The light beam incident on the entrance surface 543 a isreflected at the reflecting surface 54 b in a direction havingcomponents in both a downward direction and the subject direction. Lightquantity losses here can be reduced by the satisfaction of thetotal-reflection condition. Further, the light beam has the optical axisreflected at the reflecting surface 54 c in the viewer direction. Lightquantity losses here can be reduced by the satisfaction of thetotal-reflection condition. Note that the reflecting surfaces 54 b and54 c are tantamount to the aforesaid set PM of back-to-back tworeflecting surfaces. Further, the light beam leaves the prism 54 throughthe exit surface 54 d. The exit surface 54 d here is desirously verticalto the optical axis.

The light beam exiting the prism 54 is incident on a condenser lens 26placed near the secondary image-formation position 17. In the embodimentof FIG. 40, the condenser lens 26 is a plano-convex positive lens convexon the subject side, with its plane side substantially in alignment withthe secondary image-formation position 17.

Further, the light beam passes through a dust-preventive glass 32. Thedust-preventive glass 32 cooperates with other frame or the like toprevent dusts and other contaminants from deposition onto lens surfacesnear the secondary image-formation position 17.

Further, the light beam is subjected to lens actions at lens 27, lens 28and lens 29 forming together the eyepiece optical system (eyepiece lenssystem), exiting from a camera body. Finally, the light beam is guidedto the eye of the viewer.

FIG. 41 is a section view of the finder optical system of NumericalEmbodiment 2 corresponding to the finder optical system of FIG. 36, astaken apart along its optical axis, with its numerical data identicalwith those in Numerical Embodiment 2 according to the second aspect ofthe invention.

In FIG. 41, the respective optical members in FIG. 36 are indicated byreference numerals. Referring again to this, surface r₁ corresponds tothe primary image-formation position on the focal plane plate 12, andsurfaces r₂-r₅ correspond to the prism 13, wherein the surface r₂corresponds to the entrance surface 13 a, the surface r₃ to thereflecting surface 13 b, the surface r₄ to the reflecting surface 13 c,and the surface r₅ to the exit surface 13 d.

Surface r₆ corresponds to the mirror 14, surfaces r₇-r₈ to the lens 21,surfaces r₉-r₁₀ to the lens 22, surface r₁₁ to the aperture stop 31,surfaces r₁₂-r₁₃ to the lens 23, and r₁₄-r₁₆ to the lens 24.

Surface r₁₇ corresponds to the mirror 15, surfaces r₁₈-r₁₉ to thepositive lens 25, surface r₂₀ to the mirror 16, surfaces r₂₁-r₂₂ to thecondenser lens 26, and surface r₂₃ in alignment with the surface r₂₂ tothe secondary image-formation position 17.

Surfaces r₂₄-r₂₅ correspond to the dust-preventive glass 32; surfacesr₂₆-r₂₈, r₂₉-r₃₀ and r₃₁-r₃₂ correspond to the lenses 27, 28 and 29,respectively, which form together the eyepiece optical system; andsurface r₃₃ to the pupil of the viewer, viz., the eye point EP.

In this embodiment, the relay optical system is made up of lenses 21,22, 23, 24, 25 and 26, wherein the lens 21 is a positive meniscus lensconvex on the primary image-formation position side, the lens 22 is apositive meniscus lens convex on the primary image-formation positionside, the lens 23 is a double-concave negative lens, the lens 24 is acemented doublet consisting of a negative meniscus lens convex on theprimary image-formation position side and a double-convex positive lens,the positive lens 25 is a double-convex positive lens, and the condenserlens 26 is a plano-convex positive lens. The eyepiece optical system ismade up of lenses 27, 28 and 29, wherein the lens 27 is a cementeddoublet consisting of a double-concave negative lens and a double-convexpositive lens, the lens 28 is a double-convex positive lens, and thelens 29 is a double-convex positive.

Aspheric surfaces are used at both surfaces r₁₈ and r₁₉ of the positivelens 25 and the surface r₂₁ of the condenser lens 26 on the primaryimage-formation position side.

Note that in the numerical data enumerated later, the relations ofdiopter to the diopter control surface spaces d₂₅ and d₃₀ are shownalong with the angles of the optical axis at the reflecting surfaces 13b and 13 c and mirrors 14, 15 and 16.

FIG. 24 is a sectional view of the finder optical system of NumericalEmbodiment 3 corresponding to the finder optical system of FIG. 39, astaken apart along its optical axis, with its numerical data given later.In FIG. 42, the corresponding optical members in the arrangement of FIG.39 are indicated by reference numerals. This embodiment is basicallyidentical with Numerical Embodiment 2 of FIG. 41, except that thesurface r₅ of surfaces r₂-r₅ corresponding to the exit surface 13 d ofthe prism 13 is of convex shape. The relay optical system is made up oflenses 21, 22, 23, 24, 25 and 26, wherein the lens 21 is a positivemeniscus lens convex on the primary image-formation position side, thelens 22 is a positive meniscus lens convex on the primaryimage-formation position side, the lens 23 is a double-concave negativelens, the lens 24 is a cemented doublet consisting of a negativemeniscus lens convex on the primary image-formation position side and adouble-convex positive lens, the lens 25 is a double-convex positivelens, and the condenser lens 26 is a plano-convex positive lens. Theeyepiece optical system is made up of lenses 27, 28 and 29, wherein thelens 27 is a cemented doublet consisting of a double-concave negativelens and a double-convex positive lens, the lens 28 is a double-convexpositive lens, and the lens 29 is a plano-convex positive lens.

Four aspheric surfaces are used: one at the surface r₇ of the lens 21 onthe primary image-formation position side, two at both surfaces r₁₈ andr₁₉ of the positive lens 25, and one at the surface r₂₁ of the condenserlens 26 on the primary image-formation position side.

In the numerical data given later, the relations of diopter to thediopter control surface spaces d₂₅ and d₃₀ are shown, as in NumericalEmbodiment 2, along with the angles of the optical axis reflected at thereflecting surfaces 13 b and 13 c and the mirrors 14, 15 and 16.

In what follows, the numerical data on Numerical Embodiment 3 are given.For the symbols, etc., see Numerical Embodiments 0, 1 and 2.

Numerical embodiment: 3 (−1 diopter) r₁ = ∞ d₁ = 7.00 n_(d1) = 1.51633ν_(d1) = 64.14 (First image plane) d₂ = 10.00 n_(d2) = 1.51633 ν_(d2) =64.14 r₂ = ∞ d₃ = 22.80 n_(d3) = 1.51633 ν_(d3) = 64.14 r₃ = ∞ d₄ =10.80 n_(d4) = 1.69350 ν_(d4) = 53.21 r₄ = ∞ d₅ = 8.92 n_(d5) = 1.80400ν_(d5) = 46.57 r₅ = −66.31 d₆ = 7.88 n_(d6) = 1.84666 ν_(d6) = 23.78 r₆= ∞ d₇ = 5.03 n_(d7) = 1.71736 ν_(d7) = 29.52 r₇ = 15.05(Aspheric) d₈ =0.46 n_(d8) = 1.80400 ν_(d8) = 46.57 r₈ = 191.03 d₉ = 4.84 n_(d9) =1.52542 ν_(d9) = 55.78 r₉ = 14.60 d₁₀ = 1.84 n_(d10) = 1.52542 ν_(d10) =55.78 r₁₀ = 23.40 d₁₁ = 0.80 n_(d11) = 1.51633 ν_(d11) = 64.14 r₁₁ = ∞(Stop) d₁₂ = 1.10 n_(d12) = 1.84666 ν_(d12) = 23.78 r₁₂ = −13.71 d₁₃ =8.96 n_(d13) = 1.60311 ν_(d13) = 60.64 r₁₃ = 9.89 d₁₄ = 1.56 n_(d14) =1.78590 ν_(d14) = 44.20 r₁₄ = 1037.06 d₁₅ = 5.59 n_(d15) = 1.83400ν_(d15) = 37.16 r₁₅ = 24.71 d₁₆ = 10.41 r₁₆ = −20.86 d₁₇ = 11.73 r₁₇ = ∞d₁₈ = 5.36 r₁₈ = 26.73(Aspheric) d₁₉ = 10.41 r₁₉ = −273.06(Aspheric) d₂₀= 10.82 r₂₀ = ∞ d₂₁ = 2.97 r₂₁ = 59.97(Aspheric) d₂₂ = 0.00 r₂₂ = ∞ d₂₃= 4.14 r₂₃ = ∞ d₂₄ = 1.00 (Second image plane) d₂₅ = 7.79 r₂₄ = ∞ d₂₆ =1.37 r₂₅ = ∞ d₂₇ = 6.36 r₂₆ = −14.54 d₂₈ = 0.50 r₂₇ = 69.47 d₂₉ = 3.67r₂₈ = −20.03 d₃₀ = 3.38 r₂₉ = 284.63 d₃₁ = 3.30 r₃₀ = −38.06 d₃₂ = 23.00r₃₁ = 37.41 r₃₂ = ∞ r₃₃ = ∞ (Pupil of Observer) Aspherical Coefficients7th surface K = 0.0590 A₄ = 1.20 × 10⁻⁵ 18th surface K = −0.8962 A₄ =1.73 × 10⁻⁵ 19th surface K = −0.0551 A₄ = 1.60 × 10⁻⁵ 21th surface K =−0.2550 A₄ = 6.30 × 10⁻⁶ Diopter Movement −1 diopter −3 diopter +1diopter d₂₅ 7.79 5.58 9.96 d₃₀ 3.38 5.59 1.20 Reflection Angle ofOptical Axis r₃ 106°  r₄ 116°  r₈ 78° r₁₇ 77° r₂₀ 75°

FIG. 43 is a collection of aberration diagrams for Numerical Embodiment2, and FIG. 44 is a collection of aberration diagrams for NumericalEmbodiment 3. In these aberration diagrams, (a), (b) and (c) areindicative of spherical aberration (SA), astigmatism (AS), distortion(DT) and chromatic aberration of magnification (CC) at the time of +1diopter, −1 diopter, and −3 diopter, respectively, and “φ” and “ω” areindicative of pupil diameter and an exit angle, respectively. Note thatthe states of aberrations in the aberration diagrams are illustrative ofthose in the finder optical system after the primary image-formationplane.

1. An eyepiece optical system, comprising, in order from a side of animage being viewed, a first lens having negative refracting power, asecond lens having positive refracting power, a third lens havingpositive refracting power, and a fourth lens having positive refractingpower, wherein the eyepiece optical system satisfies the followingcondition:2.5≦f123/fA≦8  (4-1) where f123 is a composite focal length of the firstlens, the second lens and the third lens, and fA is a focal length ofthe eyepiece optical system.
 2. An eyepiece optical system, comprising,in order from a side of an image being viewed, a first lens havingnegative refracting power, a second lens having positive refractingpower, a third lens having positive refracting power, and a fourth lenshaving positive refracting power, wherein the eyepiece optical systemsatisfies the following condition:1≦f4/fA≦2  (4-2) where f4 is a focal length of the fourth lens, and fAis a focal length of the eyepiece optical system.
 3. An eyepiece opticalsystem, comprising, in order from a side of an image being viewed, afirst lens having negative refracting power, a second lens havingpositive refracting power, a third lens having positive refractingpower, and a fourth lens having positive refracting power, wherein theeyepiece optical system satisfies the following condition:0.02≦d4/fA≦0.2  (4-3) where d4 is an axial thickness of the fourth lens,and fA is a focal length of the eyepiece optical system.
 4. The eyepieceoptical system according to claim 1, wherein the image being viewed isan image or aerial image formed through an image-formation lens.
 5. Arelay type finder optical system, comprising: a relay optical systemoperable to re-form a primary image formed through a taking opticalsystem, and an eyepiece optical system operable to view an imagere-formed through the relay optical system, wherein: the eyepieceoptical system is the eyepiece optical system according to claim
 1. 6.The relay type finder optical system according to claim 1, wherein: thefourth lens remains fixed, and diopter control is implemented byintegral movement of the first lens, the second lens and the third lensin an optical axis direction.
 7. The eyepiece optical system accordingto claim 1, wherein the first lens and the second lens are mutuallycemented into a cemented doublet.
 8. The eyepiece optical systemaccording to claim 1, wherein the first lens and the second lens aremutually cemented together into a cemented doublet.
 9. The relay typefinder optical system according to claim 6, which satisfies thefollowing conditions:2.5≦f123/fA≦6.5  (4-1)′1.4≦f4/fA≦2  (4-2)′ where f123 is a composite focal length of the firstlens, the second lens and the third lens, f4 is a focal length of thefourth lens, and fA is a focal length of the eyepiece optical system.10. An imaging apparatus, comprising: a taking lens, a quick returnmirror adapted to reflect light rays leaving the taking lens, an imagingdevice operating such that when said quick return mirror is retracted,an image formed by light rays entering from the taking lens is convertedinto electrical signals, and an eyepiece optical system as recited inclaim 1, on which light rays reflected off said quick return mirror areincident.